3.8: Milky Way Galaxy - Geosciences

3.8: Milky Way Galaxy - Geosciences

On a dark, clear night, you will see a milky band of light stretching across the sky. This band is the disk of a galaxy, the Milky Way Galaxy, is our galaxy and is made of millions of stars along with a lot of gas and dust.

Shape and Size

Although it is difficult to know what the shape of the Milky Way Galaxy is because we are inside of it, astronomers have identified it as a typical spiral galaxy containing about 100 billion to 400 billion stars.

Like other spiral galaxies, our galaxy has a disk, a central bulge, and spiral arms. The disk is about 100,000 light-years across and 3,000 light-years thick. Most of the Galaxy’s gas, dust, young stars, and open clusters are in the disk. What data and evidence do astronomers find that lets them know that the Milky Way is a spiral galaxy?

  1. The shape of the galaxy as we see it.
  2. The velocities of stars and gas in the galaxy show a rotational motion.
  3. The gases, color, and dust are typical of spiral galaxies.

The central bulge is about 12,000 to 16,000 light-years wide and 6,000 to 10,000 light-years thick. The central bulge contains mostly older stars and globular clusters. Some recent evidence suggests the bulge might not be spherical, but is instead shaped like a bar. The bar might be as long as 27,000 light-years long. The disk and bulge are surrounded by a faint, spherical halo, which also includes old stars and globular clusters. Astronomers have discovered that there is a gigantic black hole at the center of the galaxy.

The Milky Way Galaxy is a significant place. Our solar system, including the Sun, Earth, and all the other planets, is within one of the spiral arms in the disk of the Milky Way Galaxy. Most of the stars we see in the sky are relatively nearby stars that are also in this spiral arm. Earth is about 26,000 light-years from the center of the galaxy, a little more than halfway out from the center of the galaxy to the edge.Just as Earth orbits the Sun, the Sun and solar system orbit the center of the Galaxy. One orbit of the solar system takes about 225 to 250 million years. The solar system has orbited 20 to 25 times since it formed 4.6 billion years ago. Astronomers have recently found that at the center of the Milky Way, and most other galaxies, is a supermassive black hole, though a black hole cannot be seen.

A new 3D map of the Milky Way flaunts our galaxy’s warped shape

Using data from an especially bright population of stars, astronomers have reconstructed the Milky Way’s peaks and valleys like never before.

The Milky Way's population of Cepheid stars suggests that our galaxy is a twisted disk. Image Credit: J. Skowron / OGLE / Astronomical Observatory, University of Warsaw

The galaxy we live in is totally bent out of shape.

At least, that’s what the latest three-dimensional map of the Milky Way has to say. By pinpointing the locations of more than 2,400 pulsing stars—including some from the outermost edges of our galaxy—scientists have charted out a stellar atlas that might give us one of the most comprehensive portraits of the Milky Way to date.

Their findings, published today in the journal Science, reveal that the spiral galaxy we Earthlings call home isn’t the flat, featureless pancake we often make it out to be. Instead, it seems to be warped into a wave that recalls a beach towel being shaken free of sand.

The new study isn’t the first to ogle the Milky Way’s curves. But getting up close and personal with our galaxy’s warp might give us clues about its history, too—and, in doing so, give us a better sense of place in our neck of the cosmic woods.

“This is important and exciting work,” says Kathryn Johnston, an astronomer studying galactic dynamics at Columbia University who was not involved in the study. “Getting a three-dimensional map is incredibly difficult…so it’s wonderful that [the researchers] have made a global map that really allows you to look across the entire galactic disk.”

Our Milky Way is thought to be a barred spiral galaxy, containing a central bar-shaped structure composed of stars. Another barred spiral galaxy called NGC1300, imaged by the Hubble Space Telescope, is shown here. Image Credit: NASA, ESA, and The Hubble Heritage Team STScI/AURA

For decades, scientists have suspected that the Milky Way suffers from a mild case of the bends. In the 1950s, astronomers tracking our galaxy’s reservoir of hydrogen gas noticed some fraying at its fringes—an observation that appeared to be corroborated by subsequent studies monitoring everything from the distribution of Milky Way’s cosmic dust to the motion of stars skittering across the skies.

But finding definitive proof of the Milky Way’s warp is no simple task. While astronomers have gotten pretty good at snapshotting galaxies in more distant regions of the universe, here on Earth, we don’t exactly have the best vantage point to get an equivalent birds-eye view of the celestial structure that surrounds us. Johnston compares the process to trying to trace the outline of a forest after being dropped into its center.

The installation of an extragalactic telescope might yet be in our (very distant) future. In the meantime, a team of researchers led by Dorota Skowron, an astronomer at the University of Warsaw in Poland, decided to blaze a path through the galactic woodland with something a little more readily available: A trail of stellar breadcrumbs, sprinkled throughout the Milky Way itself.

Quasar Microlensing Reveals Planets in Extragalactic Galaxies

Image of the gravitational lens RX J1131-1231 galaxy with the lens galaxy at the center and four lensed background quasars. It is estimated that there are trillions of planets in the center elliptical galaxy in this image. University of Oklahoma

A University of Oklahoma astrophysics team has discovered for the first time a population of planets beyond the Milky Way galaxy. Using microlensing–an astronomical phenomenon and the only known method capable of discovering planets at truly great distances from the Earth among other detection techniques–OU researchers were able to detect objects in extragalactic galaxies that range from the mass of the Moon to the mass of Jupiter.

Xinyu Dai, professor in the Homer L. Dodge Department of Physics and Astronomy, OU College of Arts and Sciences, with OU postdoctoral researcher Eduardo Guerras, made the discovery with data from the National Aeronautics and Space Administration’s Chandra X-ray Observatory, a telescope in space that is controlled by the Smithsonian Astrophysical Observatory.

“We are very excited about this discovery. This is the first time anyone has discovered planets outside our galaxy,” said Dai. “These small planets are the best candidate for the signature we observed in this study using the microlensing technique. We analyzed the high frequency of the signature by modeling the data to determine the mass.”

While planets are often discovered within the Milky Way using microlensing, the gravitational effect of even small objects can create high magnification leading to a signature that can be modeled and explained in extragalactic galaxies. Until this study, there has been no evidence of planets in other galaxies.

“This is an example of how powerful the techniques of analysis of extragalactic microlensing can be. This galaxy is located 3.8 billion light years away, and there is not the slightest chance of observing these planets directly, not even with the best telescope one can imagine in a science fiction scenario,” said Guerras. “However, we are able to study them, unveil their presence and even have an idea of their masses. This is very cool science.”

For this study, OU researchers used the NASA Chandra X-ray Observatory at the Smithsonian Astrophysical Observatory. The microlensing models were calculated at the OU Supercomputing Center for Education and Research.

Publication: Xinyu Dai and Eduardo Guerras, “Probing Planets in Extragalactic Galaxies Using Quasar Microlensing,” ApJL, 2018 doi:10.3847/2041-8213/aaa5fb

The Milky Way Galaxy

Like early explorers mapping the continents of our globe, astronomers are busy charting the spiral structure of our galaxy, the Milky Way. Using infrared images from NASA's Spitzer Space Telescope, scientists have discovered that the Milky Way's elegant spiral structure is dominated by just two arms wrapping off the ends of a central bar of stars. Previously, our galaxy was thought to possess four major arms.

The annotated artist's concept illustrates the new view of the Milky Way. The galaxy's two major arms (Scutum-Centaurus and Perseus) can be seen attached to the ends of a thick central bar, while the two now-demoted minor arms (Norma and Sagittarius) are less distinct and located between the major arms.

Our Galaxy

The major arms consist of the highest densities of both young and old stars the minor arms are primarily filled with gas and pockets of star-forming activity.

The artist's concept also includes a new spiral arm, called the "Far-3 kiloparsec arm," discovered via a radio-telescope survey of gas in the Milky Way. This arm is shorter than the two major arms and lies along the bar of the galaxy.

Our Sun lies near a small, partial arm called the Orion Arm, or Orion Spur, located between the Sagittarius and Perseus arms.

Types of galaxies

Before the 20th century, we didn't know that galaxies other than the Milky Way existed earlier astronomers had classified them as as “nebulae,” since they looked like fuzzy clouds. But in the 1920s, astronomer Edwin Hubble showed that the Andromeda “nebula” was a galaxy in its own right. Since it is so far from us, it takes light from Andromeda more than 2.5 million years to bridge the gap. Despite the immense distance, Andromeda is the closest large galaxy to our Milky Way, and it's bright enough in the night sky that it's visible to the naked eye in the Northern Hemisphere.

In 1936, Hubble debuted a way to classify galaxies, grouping them into four main types: spiral galaxies, lenticular galaxies, elliptical galaxies, and irregular galaxies.

More than two-thirds of all observed galaxies are spiral galaxies. A spiral galaxy has a flat, spinning disk with a central bulge surrounded by spiral arms. That spinning motion, at speeds of hundreds of kilometers a second, may cause matter in the disk to take on a distinctive spiral shape, like a cosmic pinwheel. Our Milky Way, like other spiral galaxies, has a linear, starry bar at its center.

Elliptical galaxies are shaped as their name suggests: They are generally round but can stretch longer along one axis than along the other, so much so that some take on a cigar-like appearance. The universe's largest-known galaxies—giant elliptical galaxies—can contain up to a trillion stars and span two million light-years across. Elliptical galaxies may also be small, in which case they are called dwarf elliptical galaxies.

Elliptical galaxies contain many older stars, but little dust and other interstellar matter. Their stars orbit the galactic center, like those in the disks of spiral galaxies, but they do so in more random directions. Few new stars are known to form in elliptical galaxies. They are common in galaxy clusters.

Lenticular galaxies, such as the iconic Sombrero Galaxy, sit between elliptical and spiral galaxies. They're called “lenticular” because they resemble lenses: Like spiral galaxies, they have a thin, rotating disk of stars and a central bulge, but they don't have spiral arms. Like elliptical galaxies, they have little dust and interstellar matter, and they seem to form more often in densely populated regions of space.

Galaxies that are not spiral, lenticular, or elliptical are called irregular galaxies. Irregular galaxies—such as the Large and Small Magellanic Clouds that flank our Milky Way—appear misshapen and lack a distinct form, often because they are within the gravitational influence of other galaxies close by. They are full of gas and dust, which makes them great nurseries for forming new stars.

3.8: Milky Way Galaxy - Geosciences

Our blue planet Earth has long been regarded to carry full of nutrients for hosting life since the birth of the planet. Here we speculate the processes that led to the birth of early life on Earth and its aftermath, finally leading to the evolution of metazoans. We evaluate: (1) the source of nutrients, (2) the chemistry of primordial ocean, (3) the initial mass of ocean, and (4) the size of planet. Among the life-building nutrients, phosphorus and potassium play a key role. Only three types of rocks can serve as an adequate source of nutrients: (a) continent-forming TTG (granite), enabling the evolution of primitive life to metazoans (b) primordial continents carrying anorthosite with KREEP (Potassium, Rare Earth Elements, and Phosphorus) basalts, which is a key to bear life (c) carbonatite magma, enriched in radiogenic elements such as U and Th, which can cause mutation to speed up evolution and promote the birth of new species in continental rift settings. The second important factor is ocean chemistry. The primordial ocean was extremely acidic (pH = 1–2) and enriched in halogens (Cl, F and others), S, N and metallic elements (Cd, Cu, Zn, and others), inhibiting the birth of life. Plate tectonics cleaned up these elements which interfered with RNA. Blue ocean finally appeared in the Phanerozoic with pH = 7 through extensive interaction with surface continental crust by weathering, erosion and transportation into ocean. The initial ocean mass was also important. The birth of life and aftermath of evolution was possible in the habitable zone with 3–5 km deep ocean which was able to supply sufficient nutrients. Without a huge landmass, nutrients cannot be supplied into the ocean only by ridge-hydrothermal circulation in the Hadean. Finally, the size of the planet plays a crucial role. Cooling of massive planets is less efficient than smaller ones, so that return-flow of seawater into mantle does not occur until central stars finish their main sequence. Due to the suitable size of Earth, the dawn of Phanerozoic witnessed the initiation of return-flow of seawater into the mantle, leading to the emergence of huge landmass above sea-level, and the distribution of nutrients on a global scale. Oxygen pump also played a critical role to keep high-PO2 in atmosphere since then, leading to the emergence of ozone layer and enabling animals and plants to invade the land.

To satisfy the tight conditions to make the Earth habitable, the formation mechanism of primordial Earth is an important factor. At first, a ‘dry Earth’ must be made through giant impact, followed by magma ocean to float nutrient-enriched primordial continents (anorthosite + KREEP). Late bombardment from asteroid belt supplied water to make 3–5 km thick ocean, and not from icy meteorites from Kuiper belt beyond cool Jupiter. It was essential to meet the above conditions that enabled the Earth as a habitable planet with evolved life forms. The tight constraints that we evaluate for birth and evolution of life on Earth would provide important guidelines for planetary scientists hunting for life in the exo-solar planets.

Graphical abstract


► Processes that led to the birth and evolution of early life on Earth evaluated. ► Source or nutrients, chemistry and initial mass of primordial ocean and size of rocky planet identified as critical. ► Guidelines for hunting life in the exo-solar planets proposed.

New Milky Way Map Reveals A Wave Of Stars In Our Galaxy’s Outer Reaches

(CNN) — A new map reveals the outskirts of the Milky Way galaxy, including a wave of stars disturbed by a small galaxy on a collision course with our own.

Data collected from the European Space Agency’s Gaia mission and NASA’s Near Earth Object Wide Field Infrared Survey Explorer has been used by astronomers to map the galactic halo and this group of stars. Their findings appear in a study published Wednesday in the journal Nature.

Our Milky Way is a galaxy with multiple spiral arms emanating from a central disk. The empty-looking halo lies outside of these swirling arms. But there may be more to the halo than meets the eye.

The halo, which hosts a small population of stars, is also thought to contain a lot of dark matter. This mysterious substance, which is invisible and has eluded scientists for decades, is thought to comprise most of the mass in the universe.

A small neighboring galaxy, known as the Large Magellanic Cloud, orbits the Milky Way. The data used to create the map revealed that, like a ship, the Large Magellanic Cloud has cut through the Milky Way’s outer halo. This disturbance has left a rippling wave of stars behind the Large Magellanic Cloud, which is in the halo.

A collision of galaxies

Currently, the Large Magellanic Cloud is 160,000 light-years from Earth, and it only has about a quarter of the mass of our giant galaxy.

Research from 2019 suggests it will catastrophically collide with our own galaxy in 2 billion years.

The impact has a chance of sending our solar system hurtling through space.

The wake created by the Large Magellanic Cloud is about 200,000 light-years to 325,000 light-years from the galactic center.

While previous research suggested its existence, this new data provides confirmation, as well as the most detailed and accurate map of the galaxy’s outskirts.

In the image, the strip in the middle represents a 360-degree view of our galaxy overlaying a map of the galactic halo. A bright wave in the bottom left of the image is the wake of stars, and to the right is the Large Magellanic Cloud and the path it is taking.

A large, light blue feature in the top right shows a high concentration of stars in our galaxy’s northern hemisphere.

Understanding dark matter

The ripple left by the dwarf galaxy’s movement is also an opportunity to study dark matter. Even though dark matter is invisible, it provides structure throughout the universe — including the foundation for galaxies.

So if the Large Magellanic Cloud can cut through the Milky Way’s halo and leave a wave of stars, the same ripple should essentially act as an outline of the dark matter.

Dark matter is essentially pulling on the Large Magellanic Cloud to slow it down, shrinking the dwarf galaxy’s orbit around the Milky Way and causing the eventual collision.

While it sounds violent, galactic collisions are what have created the massive galaxies populating our universe — and our own galaxy has previously experienced mergers before.

“This robbing of a smaller galaxy’s energy is not only why the (Large Magellanic Cloud) is merging with the Milky Way, but also why all galaxy mergers happen,” said Rohan Naidu, study co-author and a doctoral student in astronomy at Harvard University, in a statement. “The wake in our map is a really neat confirmation that our basic picture for how galaxies merge is on point!”

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Monday, July 12 - Venus Kisses Mars (after sunset)

On the evenings surrounding Monday, July 12, extremely bright Venus and much fainter Mars will meet in a very close conjunction quite low in the west-northwestern sky. While both planets have been traveling eastward in their orbits (red tracks with labelled dates:times), the faster motion of inner planet Venus will cause it to catch up to and pass slower-moving Mars from tonight to tomorrow. Look closely! Magnitude +1.84 Mars will be nearly 200 times fainter than magnitude -3.87 Venus, and positioned just 34 arc-minutes (equal to about the full moon&rsquos diameter) to the lower left of Venus. From a location with an unobstructed horizon, start to look for the planets after about 9 p.m. local time, when they&rsquoll sit a fist&rsquos diameter above the horizon. They&rsquoll set by 10 p.m. local time. Binoculars (red circle) will help &ndash but use them only after the sun has completely set. The two planets will share the view in binoculars from about July 4 to 21, but they&rsquoll only be telescope-close (yellow circle) from July 11 to 14.

When is the Milky Way Visible?

The core of the milky way is only visible about half of the year. The other half it is located beneath the horizon. In the winter months (December – February) it is not visible at all because it’s too close to the sun. In the spring (March – May), it will first become visible a few hours before sunrise. By June it will rise much earlier before midnight. The summer months (June – August) are generally the best viewing time because it will be up most of the night. By fall (September – November) the milky way will be best seen in the evening, before it sets. Twilight can brighten the sky up to 2 hours before sunrise and 2 hours after sunset, so you want to avoid those times.

The milky way was setting at the horizon by the time it got dark at Dead Horse Point, UT, in early November. [Buy Photo]

The rotation of the earth is what causes the stars to appear to move across the sky every night. But the earth does not actually take 24 hours to make a full rotation. It takes 23 hours and 56 minutes. This 4 minute difference is what causes the stars to change from night to night. Every night a given star will rise, cross the sky, or set 4 minutes earlier compared to the previous night. This change amounts to 2 hours every month. For example if the milky way rises at midnight tonight, a month from now it will be rising at 10pm. If you go out at the same time, the sky will look quite a bit different.

To get a better idea of the motion of the stars, download the free software Stellarium.

Watch the milky way rise over Lake Sugema, IA, in this time lapse video shot in the early morning hours at the end of April.

Geologic and Biological Timeline of the Earth

Astronomical and geological evidence indicates that the Universe is approximately 13,820 million years old[42], and our solar system is about 4,567 million years old. Earth's Moon formed 4,450 million years ago, just 50 million years after the Earth's formation.

Because the composition of the rocks retrieved from the Moon by the Apollo missions is very similar to rocks from the Earth, it is thought that the Moon formed as a result of a collision between the young Earth and a Mars-sized body, sometimes called Theia, which accreted at a Lagrangian point 60° ahead or behind the Earth. A cataclysmic meteorite bombardment (the Late Heavy Bombardment) of the Moon and the Earth 3,900 million years ago is thought to have been caused by impacts of planetesimals which were originally beyond the Earth, but whose orbits were destabilized by the migration of Jupiter and Saturn during the formation of the solar system. The Mars Reconnaissance Orbiter and the Mars Global Surveyor have found evidence that the Borealis basin in the northern hemisphere of Mars may have been created by a colossal impact with an object 2,000 kilometers in diameter at the time of the Late Heavy Bombardment.[20]

Approximately 4,000 million years ago, the earth was cool enough for land masses to form. The supercontinent Rodinia was formed about 1100 million years ago, and it broke into several pieces that drifted apart 750 million years ago. Those pieces came back together about 600 million years ago, forming the Pan-African mountains in a new supercontinent called Pannotia. Pannotia started breaking up 550 million years ago to form Laurasia and Gondwana. Laurasia included what are now North America, Europe, Siberia, and Greenland. Gondwana included what is now India, Africa, South America, and Antarctica. Laurasia and Gondwana rejoined approximately 275 million years ago to form the supercontinent Pangea. The break up of Pangea, which still goes on today, has contributed to the formation of the Atlantic Ocean.

(mya = million years ago)
The times are approximate and may vary by a few million years.
Precambrian Time
(4567 to 542 mya)

Hadean Eon (4567 to 4000 mya)
- 4650 mya: Formation of chondrules in the Solar Nebula
- 4567 mya: Formation of the Solar System
Sun was only 70% as bright as today.
- 4500 mya: Formation of the Earth.
Formation of the Moon
- 4450 mya: The Moon accretes from fragments
of a collision between the Earth and a planetoid
Moon's orbit is beyond 64,000 km from the Earth.[33]
Earth day is 7 hours long[34]
- Earth's original hydrogen and helium atmosphere
escapes Earth's gravity.
- 4455 mya: Tidal locking causes one side
of the Moon to face the Earth permanently.[30]
- 4280 mya: Water started condensing in liquid form.
- 3900 mya: Cataclysmic meteorite bombardment.
The Moon is 282,000 km from Earth.[34]
Earth day is 14.4 hours long[34]
- Earth's atmosphere becomes mostly
carbon dioxide, water vapor,
methane, and ammonia.
- Formation of carbonate minerals starts
reducing atmospheric carbon dioxide.
- There is no geologic record for the Hadean Eon.

Archean Eon (4000 to 2500 mya)
Eoarchean Era (4000 to 3600 mya)

- 4000 mya: The Earth's crust cooled and solidified.
- Atmospheric pressure ranged from 100 to 10 bar.
- Earth day is 15 hours long
Paleoarchean Era (3600 to 3200 mya)
Start of Plate Tectonics
- 3600 mya: Formation of first supercontinent Vaalbara.
- 3500 mya: Monocellular life started ( Prokaryotes ).
First known oxygen-producing bacteria:

cyanobacteria (blue-green algae) form stromatolites
- Oldest unambiguous microfossils date from this era.
Mesoarchean Era (3200 to 2800 mya)
- 3000 mya: Atmosphere has 75% nitrogen,
15% carbon dioxide.
- Sun brightens to 80% of current level.
- 2900 mya: Pongola glaciation occurred.
Neoarchean Era (2800 to 2500 mya)
- 2800 mya: Break up of supercontinent Vaalbara.
- Oldest record of Earth's magnetic field.
- 2700 mya: Supercontinent Kenorland formed.
- Photosynthetic organisms proliferate.

Proterozoic Eon (2500 to 542 mya)
Paleoproterozoic Era (2500 to 1600 mya)
Siderian Period (2500 to 2300 mya)
- Stable continents first appeared.
- 2500 mya: First free oxygen is found
in the oceans and atmosphere.
Banded Iron Formations
- 2400 mya: Great Oxidation Event,
also called the Oxygen Catastrophe.
Oxidation precipitates dissolved iron
creating banded iron formations.[14]
Anaerobic organisms are poisoned by oxygen.
- 2400 mya: Start of Huronian ice age
Rhyacian Period (2300 to 2050 mya)
- 2200 mya: Organisms with mitochondria
capable of aerobic respiration appear.
- 2100 mya: End of Huronian ice age
Orosirian Period (2050 to 1800 mya)
- Intensive orogeny (mountain development)
- 2023 mya: Meteor impact, 300 km crater
Vredefort, South Africa [9]
- 2000 mya: Solar luminosity is 85% of current level.
- Oxygen starts accumulating in the atmosphere
- 1850 mya: Meteor impact, 250 km crater
Sudbury, Ontario, Canada [9]
Statherian Period (1800 to 1600 mya)
- 1800 mya: Supercontinent Columbia (Nuna) formed.
- Complex single-celled life appeared.
- Abundant bacteria and archaeans.
Mesoproterozoic Era (1600 to 1000 mya)
Calymmian Period (1600 to 1400 mya)
- Photosynthetic organisms continue to proliferate.
- Oxygen builds up in the atmosphere above 10%.
- Formation of ozone layer starts blocking
ultraviolet radiation from the sun.
- 1600 mya: Eukaryotic (nucleated) cells appear.
Origin of ancestor of all animals, plants and fungi

Ectasian Period (1400 to 1200 mya)
- Green (Chlorobionta) and red (Rhodophyta) algae abound.
Stenian Period (1200 to 1000 mya)
- 1200 mya: Spore/gamete formation indicates
origin of sexual reproduction.[36]
- 1100 mya: Formation of the supercontinent Rodinia

Neoproterozoic Era (1000 to 542 mya)
Tonian Period (1000 to 850 mya)
- 1000 mya: Multicellular organisms appear.
- 950 mya: Start of Stuartian-Varangian ice age
- 900 mya: Earth day is 18 hours long.
The Moon is 350,000 km from Earth.[31]
Cryogenian Period (850 to 630 mya)

- 750 mya: Breakup of Rodinia
- 650 mya: * Mass extinction of 70% of dominant sea plants
due to global glaciation ("Snowball Earth" hypothesis).
The Moon is 357,000 km from Earth.[31]
Ediacaran (Vendian) Period (630 to 542 mya)
- 600 mya: Formation of the supercontinent Pannotia
Earth day is 20.7 hours long.[35]
- 590 mya: Meteor impact, 90 km crater
Acraman, South Australia

- 580 mya: Soft-bodied organisms developed:
Jellyfish, Tribrachidium, and Dickinsonia appeared.
- 570 mya: End of Stuartian-Varangian ice age
Shelled invertebrates appeared
- 550 mya: Pannotia fragmented into Laurasia and Gondwana

Phanerozoic Eon
(542 mya to present)

Paleozoic Era (542 to 251 mya)
Cambrian Period (542 to 488.3 mya)
- Abundance of multicellular life.
- Most of the major groups of animals first appear
Tommotian Stage (534 to 530 mya)
- 510 mya: Vertebrates appeared in the ocean.
Solar brightness was 6% less than today.
Ordovician Period (488.3 to 443.7 mya)

- diverse marine invertebrates, such as trilobites,
became common
- First green plants and fungi on land.
- Fall in atmospheric carbon dioxide.
- 450 mya: Start of Andean-Saharan ice age.
- 443 mya: Glaciation of Gondwana.
* Mass extinction of many marine invertebrates.
Second largest mass extinction event.
49% of genera of fauna disappeared.
Silurian Period (443.7 to 416 mya)

- 420 mya: End of Andean-Saharan ice age.
- Stabilization of the earth's climate
- Land plants and coral reefs appeared
- First fish with jaws - sharks
- Insects (spiders, centipedes), and plants appear on land
Devonian Period (416 to 359.2 mya)

- Ferns and seed-bearing plants (gymnosperms) appeared
- Formation of the first forests
- Earth day is

21.8 hours long.
- 400 mya: Land animals appeared, wingless insects
- 376 mya: Viluy Traps Large Igneous Province volcanic eruption
- 375 mya: Vertebrates with legs, such as Tiktaalik appeared.
- Atmospheric oxygen level is about 16%

- First amphibians appear.
- 374 mya: * Mass extinction of 70% of marine species.
This was a prolonged series of extinctions
occurring over 20 million years.
Evidence of anoxia in oceanic bottom waters,
and global cooling. Surface temperatures dropped
from about 93°F (34°C) to about 78°F (26°C)
- 370 mya: First trees appeared
- 359 mya: Meteor impact, 40 km crater
Woodleigh, Australia
Carboniferous Period (359.2 to 299 mya)
Mississippian Epoch (359.2 to 318.1 mya)
(Lower Carboniferous)
- 350 mya: Beginning of Karoo ice age.

- Large primitive trees develop
- First winged insects
- Forests consist of ferns, club mosses, horsetails, and gymnosperms.
- Oxygen levels increase
- 324 mya: Synapsid vertebrates, the ancestors of mammals,
appear on land.
- Seas covered parts of the continents
- Animals laying amniote eggs appear (318 mya)
Pennsylvanian Epoch (318.1 to 299 mya)
(Upper Carboniferous)
- 300 mya: First reptiles (diapsids) appeared.
They were ancestors of crocodiles, dinosaurs and pterosaurs.
- Atmospheric oxygen levels reach over 30%
- Earth day is

22.4 hours long.
The Moon is 375,000 km from Earth.[31]

- Giant arthropods populate the land
- Transgression and regression of the seas
caused by glaciation
- Deposits of coal form in Europe, Asia,
and North America
Permian Period (299 to 251 mya)

- 275 mya: Formation of the supercontinent Pangea

- Conifers and cycads first appear
- Earth is cold and dry

- 167 mya: Meteor impact, 80 km crater
Puchezh-Katunki, Russia [9]
- 166 mya: Evolutionary split of monotremes from primitive mammals

- 150 mya: First birds like Archaeopteryx appear
- 148 mya: Evolutionary split between
marsupial and eutherian mammals
- 145 mya: Meteor impact, 70 km crater
Morokweng, South Africa [9]
Cretaceous Period (145.5 to 65.5 mya)
- Period of Active Crust Plate Movements
- 133 mya: Meteor impact, 55 km crater
Tookoonooka, Australia [9]
- 125 mya: Africa and India separate from Antarctica

- Flowering plants (angiosperms) appeared
Montsechia vidalli was one of the first angiosperms.
- 120 mya: Global warming event starts
Carbon dioxide levels were 550 to 590 ppm [27]
- Proliferation of single-cell organisms (diatoms, dinoflagellates,
and calcareous nannoplankton) changed ocean composition.
- 116 mya: First birds with beaks without teeth.
- 110 mya: Crocodiles appeared
- Snakes evolved during the mid-Cretaceous
- 105 mya: South America breaks away from Africa
- Formation of the Atlantic Ocean

Tertiary Period (65.5 to 2.58 mya)

Paleocene Epoch (65.5 to 55.8 mya)
- 63 mya: End of Deccan Traps volcanic eruptions in India
- Flowering plants become widespread.
- 70% of new bird lineages evolved within five million years
after the extinction of the dinosaurs.
- Social insects achieve ecological dominance.

- Appearance of placental mammals
(marsupials, insectivores, lemuroids, creodonts)
- 60 mya: Earliest known ungulate (hoofed mammal)
- The Laramide orogeny starts forming the Rocky Mountains
and draining the Western Interior Seaway.
- First appearance of grasses.[43]
- 55.8 mya: Major global warming episode (PETM)[39]
North Pole temperature averaged 23°C (73.4°F),
CO2 concentration was 2000 ppm.
Eocene Epoch (55.8 to 33.9 mya)
- 50 mya: India meets Asia forming the Himalayas
- 45 mya: Earth day is 24 hours long.
The Moon is 378,000 km from Earth.[32]
- Modern mammals appear
rhinoceros, camels, early horses and
lemur-like primates appear
- 35.6 mya: Meteor impacts, 90 and 100 km craters
Chesapeake Bay, Virginia, USA, and
Popigai, Russia [9,10]
- Global temperature dropped 10°C during the Eocene.
- 34 mya: Global cooling creates
permanent Antarctic ice sheet [21]
Oligocene Epoch (33.9 to 23.03 mya)
- 30 mya: Australia and South America separate from Antarctica

- First elephants with trunks
- 27.8 mya: La Garita, Colorado supervolcanic eruption

Miocene Epoch (23.03 to 5.3 mya)
- African-Arabian plate joined to Asia
- 21 to 14 mya: Miocene warming period
- 14 mya: Circum-polar ocean circulation builds up Antarctic ice cap.
- 14 to 6 mya: Global temperature drops by 4°C.
- First raccoons appear.
- Drying of continental interiors
- Forests give way to grasslands
- 6 mya: Upright walking (bipedal) hominins appear
Pliocene Epoch (5.3 to 2.58 mya)
- 4.4 mya: Appearance of Ardipithecus, an early hominin genus.
- 4 mya: North and South America join at the Isthmus of Panama.
Animals and plants cross the new land bridge.
Ocean currents change in the newly isolated Atlantic Ocean.

- 3.9 mya: Appearance of Australopithecus, genus of hominids.
- 3.7 mya: Australopithecus hominids inhabit Eastern and Northern Africa.
- 3 mya: Formation of Arctic ice cap.
- Accumulation of ice at the poles
- Climate became cooler and drier.
- Spread of grasslands and savannas
- Rise of long-legged grazing animals
Quaternary Period (2.58 mya to today)
Pleistocene Epoch (2.58 mya to 11,400 yrs ago)
- Several major episodes of global cooling, or glaciations

- 2.4 mya: Homo habilis appeared
- 2.1 mya: Yellowstone supervolcanic eruption
- 2 mya: Tool-making humanoids emerge.
Beginning of the Stone Age.
- 1.7 mya: Homo erectus first moves out of Africa
- 1.3 mya: Yellowstone supervolcanic eruption
- 1.3 mya to 820,000 yrs ago: Sherwin Glaciation
- Presence of large land mammals and birds
- 790,000 yrs ago: First use of fire by hominds[40]
- 700,000 yrs ago: Human and Neanderthal lineages start to diverge genetically.
- 680,000 to 620,000 yrs ago: Günz/Nebraskan glacial period
- 640,000 yrs ago: Yellowstone supervolcanic eruption
- 530,000 yrs ago: Development of speech in Homo Heidelbergensis[15]
- 455,000 to 300,000 yrs ago: Mindel/Kansan glacial period
- 400,000 yrs ago: Hominids hunt with wooden spears
and use stone cutting tools.
- 370,000 yrs ago: Human ancestors and Neanderthals
are fully separate populations.

- 300,000 yrs ago: Hominids routinely use controlled fires
- 230,000 yrs ago: Neanderthal man spreads through Europe
- 200,000 to 130,000 yrs ago: Riss/Illinoian glacial period
- 160,000 yrs ago: Homo sapiens appeared.
Origin of human female lineage.[3]
- 125,000 yrs ago: Eemian stage or Riss/Würm interglacial period.
Hardwood forests grew above the Arctic Circle.
Melting ice sheets increased sea level by 6 meters (20 feet)
- 110,000 yrs ago: Start of Würm/Wisconsin glacial period
- 105,000 yrs ago: Stone age humans forage for grass seeds such as sorghum.
- 80,000 yrs ago: Non-African humans interbreed with Neanderthals[28]
- 74,000 yrs ago: Toba volcanic eruption
releases large volume of sulfur dioxide
- 71,000 yrs ago: Invention of the bow and arrow.[41]
- 70,000 yrs ago: Tahoe glacial maximum
glaciers cover Canada and northern US.
- 55,000 yrs ago: Humans colonize Australia
- 46,000 yrs ago: Australia becomes arid,
bush fires destroy habitat, and megafauna die off.

- 40,000 yrs ago: Cro-Magnon man appeared in Europe.
- 28,000 yrs ago: Neanderthals disappear from fossil record.[29]
- 26,500 yrs ago: Taupo supervolcanic eruption in New Zealand.
- 22,000 yrs ago: Tioga glacial maximum
sea level was 130 meters lower than today.
- 20,000 yrs ago: Invention of fired ceramic pottery.
- 19,000 yrs ago: Antarctic sea ice starts melting.[22]
- 15,000 yrs ago: Bering land bridge between Alaska and Siberia
allows human migration to America.

- 12,900 yrs ago: Comet impact by Great Lakes [23, 24, 25]
Extinction of American megafauna such as the mammoth
and sabretooth cat (Smilodon), as well as the end of Clovis culture
- 12,000 yrs ago: Construction of Göbekli Tepe in Turkey.
- 11,400 yrs ago: End of Würm/Wisconsin glacial period.
Sea level rises by 91 meters (300 ft)
Holocene Epoch (11,400 years ago to today)
- Development of agriculture
- Domestication of animals.
- 9,000 yrs ago: Metal smelting started
- 5,500 yrs ago: Invention of the wheel
- 5,300 yrs ago: The Bronze Age
- 5,000 yrs ago: Development of writing

- 4,500 yrs ago: Pyramids of Giza
- 3,300 yrs ago: The Iron Age
- 2,240 yrs ago: Archimedes develops formula
for the volume of a sphere.
- yrs ago: Start of the Common Era (CE)
- 1781 CE: James Watt patented a steam engine
that powered the Industrial Revolution.
- 1880 CE: Beginning of commercial use of electricity.
- 1945 CE: Atomic age - First nuclear explosion
- 1957 CE: Space age - Launch of Sputnik
Man-made satellite orbits the Earth.

- 1969 CE: Humans walk on the surface of the Moon.
- 1969 CE: Start of the Internet global information network.

Extinction Events
There have been five major mass extinctions events: the terminal Ordovician (443 mya), Late Devonian (374 mya), terminal Permian called the "Great Dying" (251 mya), terminal Triassic (201), and terminal Cretaceous called the K/T event (65.5 mya). The Ordovician extinction was caused by global cooling and glaciation. The other four are associated with prolonged volcanism in large igneous provinces, although a meteorite impact also contributed to the K/T event.

Humans as agents of environmental change
Some scientists have tried to correlate the migration of humans to America with the extinction of the megafauna of the Pleistocene Epoch while others feel that weather changes brought about by the explosion of an asteroid or comet over North America might have been responsible.[25] There is no doubt that human activities can have a substantial impact on the environment and native species. The dodo, a flightless bird indigenous to Mauritius, became extinct in the late 17th century from massive hunting and the introduction of animals such as dogs, pigs, and cats. The Passenger Pigeon went from being the most common bird in North America to extinction by the end of the 19th century due to hunting and loss of habitat by deforestation. Overfishing the costal waters of California in 1945 produced 235,000 tons of fish, but in 1948 only 15,000 tons of fish were caught which led to the collapse of Cannery Row. The Dust Bowl was a man-made ecological disaster caused by deep plowing of the top soil of the Great Plains which destroyed native grasses whose roots had protected the soil from erosion. Drought and wind created a period of severe dust storms between 1930 and 1936. Soils that had been fertile became incapable of growing crops after the top soil was blown away. The contemporary destruction of tropical forests by the logging industry and the large-scale clearing of forests to plant commercial crops is already having harmful ecological effects that are likely to become worse if non-sustainable practices continue to be used.

The Earth's near-term future
Human industrial activity that relies on burning fossil fuels, such as coal and petroleum products, has been generating the greenhouse gases carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), in large quantities since about 1750. The chart below shows the levels of atmospheric carbon dioxide during the last millennium and its sharp rise during the last century.[2] Atmospheric models predict that elevated greenhouse gases will cause global warming and influence weather patterns that will melt polar ice and destroy the habitat of animals such as the polar bear. The increase of global temperatures will also reduce the amount of snow deposited on mountains thus decreasing the flow of water in rivers which are now used for navigation, irrigation, and as sources of potable water. Carbon dioxide will also increase the acidity of sea water and threaten coral reefs and shell-building oceanic life forms.

As of February 2016, the monthly average level of carbon dioxide was 404.02 ppm at the National Oceanic & Atmospheric Administration (NOAA) laboratory in Mauna Loa, Hawaii, and the level continues to increase steadily. In the following image, the dashed red line represents the monthly mean values of CO2 with the points centered on the middle of each month. The black line represents the same, after correction for the average seasonal cycle.

Analysis of core sediments in the Arctic Circle indicate that 55 million years ago, the carbon dioxide concentration was 2,000 ppm and the North Pole's temperature averaged 23°C (73.4°F) compared to a mean annual temperature of -20°C today.[4] Satellite images by NASA show approximately a 20% reduction in the Earth's minimum ice cover between 1979 and 2003.[5] Arctic perennial sea ice has been decreasing at a rate of 9% every ten years. At this rate, the summertime Arctic Ocean will be ice-free before the year 2100.

There is a large amount of water stored as ice over the landmasses of Greenland and Antarctica. If the ice sheets melt, the resulting rise in global sea level will flood many coastal areas around the world. The Greenland ice sheet contains enough water to increase the global sea level by 24 feet (7.3 meters), the West Antarctic ice sheet could raise sea level by 19 feet (5.8 meters), and the East Antarctic ice sheet could raise the sea level globally by 170 feet (51.8 meters).[12] The combined effect of melting all the ice on Greenland and Antarctica would result in a sea level rise of 213 feet (65 meters).

Using computer models, scientists at the University of Arizona Department of Geosciences have created maps that show areas susceptible to rises in sea level (in red). The following map shows that a 6-meter (20-foot) rise would flood Miami, Fort Lauderdale, Tampa, and the entire Florida coastline, as well as parts of Orlando and other inland areas. Most of the city of New Orleans, Louisiana will disappear under water if the sea rises six meters. Some scientists have warned that by the year 2200, at the current rate of greenhouse gas emissions from human activities, the atmospheric levels of carbon dioxide, methane, and nitrous oxide will be at the same levels associated with mass-extinction events in the Earth's past.[8]

The Earth's long-term future
The future of the Earth is linked to the fate of the Sun. The Sun is halfway through its life cycle and will exhaust its supply of hydrogen fuel in around 4,000 million years. As the Sun cools, its core will collapse and its atmosphere will expand transforming the Sun into a red giant star. The swelling Sun will engulf the planets closest to it, and the Earth will be completely vaporized. The Sun will die in several stages. When its core crashes inwards, it will start fusing helium atoms into carbon. When the helium supply runs out, the center will collapse again and form a white dwarf star that will become dimmer until its light finally fades. The final collapse of stars which are a few times larger than the Sun ends in a massive supernova explosion that leaves behind a rapidly spinning neutron star.

Long before the Sun becomes a white dwarf, 2,000 million years from now, our Milky Way Galaxy is predicted to collide with the Andromeda Galaxy.[13] The collision will take place for several million years and result in one combined super galaxy named Milkomeda. The sun may become part of the Andromeda system during the collision and could eventually end up far away from the new merged galactic center. The Earth may also eventually lose its Moon. Scientists using the laser ranging retroreflector positioned on the Moon in 1969 by the Apollo 11 astronauts have determined that the Moon is receding from Earth at a rate of about 3.8 centimeters per year.

(my = millions of years)
Earth's Long-Term Future
+200 years: Possible global warming event caused by anthropogenic carbon dioxide (CO2)[8]
+1500 my: Sun is about 6000 million years old and 15% brighter than today.
+2000 my: Milky Way Galaxy starts colliding with Andromeda Galaxy.[13]
+3000 my: Solar system becomes part of the new Milkomeda Galaxy.
+4000 my: Sun is about twice as bright as today and its radius is 40% greater.
Sun starts to exhaust its supply of hydrogen.
+5000 my: Sun starts changing into a red giant star, 3 times its present size.[18]
Earth is engulfed by the red giant Sun.
+10000 my: Red giant Sun collapses and becomes a white dwarf.
+20000 my: White dwarf Sun becomes a black dwarf.

Age - An age is a unit of geological time shorter than an epoch, usually lasting several million years.

Anthropocene - A proposed era denoting the time when human activity started having a global impact on the Earth's surface, atmosphere and hydrosphere.

Archean, Archaean - An eon of geologic time extending from about 4000 to 2500 million years ago. Derived from the Greek archaios meaning "ancient". The Archean eon is divided into four eras: Eoarchean, Paleoarchean, Mesoarchean, and Neoarchean.

Cambrian - The first period of the Paleozoic Era, during which most modern animal phyla developed. The name derives from Medieval Latin Cambria "Wales".

Cenozoic, Caenozoic, Cainozoic - The current geologic era, which began 65.5 million years ago and continues to the present. The word comes from the Greek kainos "new" + zoe "life".

Cretaceous - A Period from 145 to 65.5 million years ago divided into two epochs:
The Early Cretaceous Epoch had six Ages: Cenomanian, Turonian, Coniacian, Santonian, Campanian, and Maastrichtian.
The Late Cretaceous Epoch had six Ages: Berriasian, Valanginian, Hauterivian, Barremian, Aptian, and Albian.

Eocene Epoch - An epoch from 54.8 to 33.9 million years ago with four Ages: Ypresian, Lutetian, Bartonian, and Priabonian.

Eon - A primary division of geologic time lasting over 500 million years, four of which have been defined: Hadean, Archean, Proterozoic, and Phanerozoic. Eons are divided into Eras, which are in turn divided into Periods, Epochs and Ages.

Epoch - A division of geologic time lasting tens of millions of years. Epochs are subdivisions of geologic periods.

Era - A division of geologic time of several hundred million years in duration. An era is smaller than an eon and longer than a period.

Geologic Time Scale - A categorization of geological events based on successively smaller time spans: eons, eras, periods, epochs, and ages.

Hadean - The earliest eon in the history of the Earth from the first accretion of planetary material until the date of the oldest known rocks. The name "Hadean" derives from the Greek Hades "Hell".

Holocene - An epoch starting 11,400 years ago to today. From holo- "whole" + Greek kainos "new".

Jurassic - A Period from 200 to 145 million years ago divided into three epochs:
The Early Jurassic Epoch has four Ages: Hettangian, Sinemurian, Pliensbachian, and Toarcian.
The Middle Jurassic Epoch has four Ages: Aalenian, Bajocian, Bathonian, and Callovian.
The Late Jurassic Epoch has three Ages: Oxfordian, Kimmeridgian, and Tithonian.

Mesoproterozoic - an era with three periods: Calymmian, Ectasian, and Stenian.

Mesozoic - An era of time during the Phanerozoic eon lasting from 251 million years ago to 65.5 million ago. Derived from the Greek mesos "middle" + zoe "life".

Miocene Epoch - An epoch from 23.03 to 5.3 million years ago with six Ages: Aquitanian, Burgidalian, Langhian, Serravalian, Tortonian, and Messinaian. The name is derived from Greek meiōn "less" + kainos "new".

Neogene - A period from 23.03 to today. This is the new name given to the time starting from the Miocene Epoch to today.

Neoproterozoic - An era with three periods: Tonian, Cryogenian, and Ediacaran.

Oligocene Epoch - An epoch from 33.9 to 23.03 million years ago with two Ages: Rupelian and Chattian. Derived from oligo- "few" + Greek kainos "new".

Paleocene, Palaeocene Epoch - An epoch from 65.5 to 54.8 million years ago with three Ages: Danian, Selandian, and Thanetian.

Paleogene - A period from 65.5 to 23.03 million years ago. This is the new name given to the first portion of the Tertiary Period.

Paleoproterozoic - an era with four periods: Siderian, Rhyacian, Orosirian, and Statherian.

Paleozoic, Palaeozoic - An era of geologic time lasting from 542 to 248 million years ago. Derived from the Greek palai "long ago, far back" + zoe "life".

Period - A division of geologic time lasting tens of millions of years which shorter than an era and longer than an epoch.

Phanerozoic - The most recent eon of geologic time beginning 542 million years ago and continuing to the present. Derived from the Greek phaneros "visible" + zoe "life".

Pleistocene - An epoch from 2.58 mya to 11,400 years ago. Derived from Greek pleistos "most" + kainos "new".

Pliocene - An epoch from 5.3 to 2.58 million years ago with two Ages: Zanclean and Piacenzian. Derived from Greek pleiōn "more" + kainos "new".

Precambrian - Geologic time from the beginning of the earth to the beginning of the Cambrian Period of the Paleozoic Era.

Proterozoic - The geologic eon lying between the Archean and Phanerozoic eons, beginning about 2500 and ending 542 million years ago. Derived from the Greek proteros "earlier" + zoe "life". The Proterozoic eon is divided into the Paleoproterozoic era, Mesoproterozoic era, and Neoproterozoic era.

Quaternary - An informal sub-era from 2.58 or 1.8 mya to today. The Quaternary is traditionally associated with the Holocene and Pleistocene, but an alternative definition sets its start during the cycle of glacials and interglacials around 2.6 mya.

Stage - A succession of rock strata laid down in a single age on the geologic timescale.

Tertiary - An informal sub-era from 65.5 to 2.58 or 1.8 million years ago, depending on how the Quaternary is defined. The Tertiary overlaps with the Neogene Period and is divided into five epochs:
Paleocene, Eocene, Oligocene, Miocene, and Pliocene.

Triassic - A Period from 251 to 200 million years ago divided into three epochs:
The Early Triassic Epoch has two Ages: Induan and Olenekian.
The Middle Triassic Epoch has two Ages: Anisian and Ladinian.
The Late Triassic Epoch has three Ages: Carnian, Norian, and Rhaetian.


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Frequent misspellings of geologic terms and Evolutionary periods of the Earth:
creataceous, cretaceus, cretacous, jurassique, jurasik, jurasic, jurossic, myscene, myocene, myoscene, phanaerozoic, triasic

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