# How many stars are between Milky way and Andromeda?

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Are there some stars between those two galaxies? In photos is only black.

And if so, how many we could find there?

The stars between galaxies are called "intergalactic stars", and one study (2012) claims to have identified about 675 of them between the Milky Way and Andromeda:

Now, Vanderbilt astronomers report in the May issue of the Astronomical Journal that they have identified a group of more than 675 stars on the outskirts of the Milky Way that they argue are hypervelocity stars that have been ejected from the galactic core. They selected these stars based on their location in intergalactic space between the Milky Way and the nearby Andromeda galaxy and by their peculiar red coloration.

-"Rogue stars ejected from the galaxy found in intergalactic space", Research News @Vanderbilt (2012-04-30)

## When will the Milky Way and Andromeda galaxies collide?

Boom! Future motions of the Milky Way and Andromeda galaxies show them on a collision course. Meanwhile, the 3rd major galaxy in our Local Group – the Triangulum galaxy – is likely to give the collision a wide berth. Image via ESA/Gaia/DPAC.

Astronomers have said for some time that the nearby Andromeda galaxy – also known as M31, nearest large spiral galaxy to our home Milky Way – will one day collide with the Milky Way. On February 7, 2019, the European Space Agency (ESA) provided an update on the newest insights about this impending collision, based on data from its Gaia satellite. Across the course of 2018, astronomers announced multiple, very interesting discoveries about our galaxy, based on Gaia’s second data release last April. Now Gaia has looked beyond the Milky Way, at the motions of stars within both the Andromeda galaxy and the Triangulum galaxy (aka M33), which is the third large galaxy in our Local Group. The data reveal some surprises about the Andromeda galaxy’s collision course with the Milky Way.

The first surprise is a new estimate for when the collision will occur. Astronomers thought it would happen some 3.9 billion years from now. But the astronomers who studied Gaia’s data said they now believe it’ll happen 600 million years later than previously estimated, perhaps 4.5 billion years from now. What’s more, they said, the Andromeda galaxy is:

… likely to deliver more of a glancing blow to the Milky Way than a head-on collision.

These results were published February 7 in the peer-reviewed Astrophysical Journal. Astronomer Roeland van der Marel of the Space Telescope Science Institute in Baltimore – who led the study – commented:

We needed to explore the galaxies’ motions in 3D to uncover how they have grown and evolved, and what creates and influences their features and behavior.

We were able to do this using the second package of high-quality data released by Gaia.

An all-sky view of our Milky Way galaxy and neighboring galaxies, based on measurements of nearly 1.7 billion stars in Gaia’s 2nd data release. The map shows the density of stars observed by Gaia in each portion of the sky from July 2014 to May 2016. Image via ESA/Gaia/DPAC. Read more about this image.

Gaia does what is called astrometry. Its job is to scan the sky repeatedly, observing each of its targeted billion-plus stars an average of 70 times over its five-year mission. Again and again and again, Gaia will acquire data points on the positions of stars in the Milky Way, and now in the Andromeda and Triangulum galaxies, too. We know that stars move through space. Gaia will tell us, exactly, how they moved during that five-year period.

It may not sound very dramatic. But it is. That much knowledge about star motions – actual data on the motions of more than a billion stars – is unprecedented in the history of astronomy. That is why there have been so many astounding discoveries from Gaia already, like this one and this one and this one.

Ultimately, Gaia’s data will be used to build the most precise 3D map of the stars in the nearby universe, ever. A statement from ESA explained:

Previous studies of the Local Group have combined observations from telescopes including the NASA/ESA Hubble Space Telescope and the ground-based Very Long Baseline Array to figure out how the orbits of Andromeda and Triangulum have changed over time. The two disk-shaped spiral galaxies are located between 2.5 and 3 million light-years from us, and are close enough to one another that they may be interacting.

Two possibilities emerged: either Triangulum is on an incredibly long six-billion-year orbit around Andromeda but has already fallen into it in the past, or it is currently on its very first infall.

Each scenario reflects a different orbital path, and thus a different formation history and future for each galaxy.

That was where things stood until Gaia came along.

Mark Fardal, also of Space Telescope Science Institute, is second author on the new paper. He explained:

We combed through the Gaia data to identify thousands of individual stars in both galaxies, and studied how these stars moved within their galactic homes

While Gaia primarily aims to study the Milky Way, it’s powerful enough to spot especially massive and bright stars within nearby star-forming regions – even in galaxies beyond our own.

The stellar motions measured by Gaia not only reveal how each of the galaxies moves through space, but also how each rotates around its own spin axis, ESA said, adding:

A century ago, when astronomers were first trying to understand the nature of galaxies, these spin measurements were much sought-after, but could not be successfully completed with the telescopes available at the time.

The sharpest-ever view of the Triangulum galaxy – aka M33 – via the Hubble Space Telescope. This image is a composite of about 54 different pointings with Hubble’s Advanced Camera for Surveys. It’s the second-largest image ever released by Hubble. Read more about this image. Want to see Hubble’s largest image ever? Look at the image below this one.

It took an observatory as advanced as Gaia to finally do so. For the first time, we’ve measured how M31 and M33 rotate on the sky. Astronomers used to see galaxies as clustered worlds that couldn’t possibly be separate ‘islands’, but we now know otherwise.

It has taken 100 years and Gaia to finally measure the true, tiny, rotation rate of our nearest large galactic neighbor, M31. This will help us to understand more about the nature of galaxies.

Sharpest-ever view of the Andromeda galaxy, via the Hubble Space Telescope. You’d need more than 600 HD TV screens to display the whole image, which is best appreciated using this zoom tool. The Hubble Telescope has provided sharp views of the Andromeda and Triangulum galaxies. But Gaia was needed to understand the motions of stars within them. Read more about this image.

Bottom line: An analysis of data from Gaia’s second data release revealed an longer-than-expected timeline for the impending collision between our Milky Way galaxy and the neighboring Andromeda galaxy. The data also show that the collision is likely to be a glancing blow, rather than the head-on crash that had been expected previously.

## When Andromeda Meets Milky

So what will happen in 4.5 billion years when Andromeda and the Milky Way meet?

First of all, the meeting will take hundreds of millions of years to conclude, if not billions. So it’s unlikely than any civilization going through a galaxy merger and surviving it can really come to grips with it. And in 4.5 billion years, our own Sun will be a red giant, and there will likely be no humans or anything else left alive on Earth. But, if there are some future, distant relatives of ours alive at that time, somewhere in the Milky Way, this is what they might experience, according to NASA.

Phase One: As the Milky Way and Andromeda approach one another, Andromeda will grow ever larger in the sky. It’ll look like an eerie, glowing sword of light.

Phase Two: As they get close enough, giant molecular clouds measuring tens or hundreds of light years across will become compressed. Millions of bright blue stars will burst into life, lighting up the sky and creating new constellations.

Phase Three: The stream of stars that makes up the Milky Way in our night sky will become disrupted and jumbled. Gas, dust, and new stars will remake our night sky. Many of the new stars will be massive, and will live a short time before exploding as supernovae. These explosions will shape the fate of any life on any worlds in their vicinity.

Phase Four: On its first pass, Andromeda will swing past the Milky Way. But then after maybe 100 million years or so, it will make a U-turn and the two galaxies will merge again. This will compress the molecular clouds again, triggering yet another round of robust star-birth. And many of those new stars will again be supernovae, so their will be another wave of massive explosions. After this second round of supernovae, their stellar winds will blow away much of the remnant gas and dust that forms new stars.

Phase Five: The two galaxies will settle down and form one elliptical galaxy. Any evidence of the two spiral galaxies that formed the new elliptical galaxy will be gone. Chances are humanity will be long gone, and any future astronomers gazing at the new galaxy will have no idea that we were once here, looking out at the universe and striving to understand it.

Back in the 1940s, a Swedish astronomer wondered what would happen if galaxies collided. His name was Erik Holmberg, and he constructed an analog computer with 200 light bulbs to simulate galactic encounters. Based on his work, he predicted that galaxies could indeed collide, and that eventually their mutual gravity would slow them down and they would merge into one.

Mostly, he was ignored, or his idea was snubbed. The idea seemed far-fetched, and his light-bulb computer seemed a fanciful invention.

Eventually, the idea gained traction and better telescopes caught these galaxies in the act. Now, we know better. We know that galaxy mergers play an important role in shaping the universe, even though we don’t know the whole picture yet.

## Andromeda–Milky Way Collision:

In 1929, Edwin Hubble revealed observational evidence which showed that distant galaxies were moving away from the Milky Way. This led him to create Hubble’s Law, which states that a galaxy’s distance and velocity can be determined by measuring its redshift – i.e. a phenomena where an object’s light is shifted toward the red end of the spectrum when it is moving away.

However, spectrographic measurements performed on the light coming from Andromeda showed that its light was shifted towards the blue end of the spectrum (aka. blueshift). This indicated that unlike most galaxies that have been observed since the early 20th century, Andromeda is moving towards us.

In 2012, researchers determined that a collision between the Milky Way and the Andromeda Galaxy was sure to happen, based on Hubble data that tracked the motions of Andromeda from 2002 to 2010. Based on measurements of its blueshift, it is estimated that Andromeda is approaching our galaxy at a rate of about 110 km/second (68 mi/s).

At this rate, it will likely collide with the Milky Way in around 4 billion years. These studies also suggest that M33, the Triangulum Galaxy – the third largest and brightest galaxy of the Local Group – will participate in this event as well. In all likelihood, it will end up in orbit around the Milky Way and Andromeda, then collide with the merger remnant at a later date.

Images from Hubble’s ACS in 2004 and 2005 show four examples of interacting galaxies (at various stages in the process) far away from Earth. Credit: NASA/ESA/J. Lotz, STScI/M. Davis, University of California, Berkeley/A. Koekemoer, STScI.

## How many stars are between Milky way and Andromeda? - Astronomy

You probably already know that the universe is big but most people do not realize how B I G it really is. Many astronomy classes start off with a tour of the universe based on the excellent short film called Powers of Ten by Charles and Ray Eames (link appears in a new window) or a recent "cover" of it such as the Cosmic Eye (link appears in a new window). The film starts with a man and woman in a city park and then expands the field of view by ten times every ten seconds until it reaches the bounds of the observable universe. After zooming back to the man and woman in the park, the field of view is reduced by ten times every ten seconds until one proton in a carbon atom in the man's hand fills the screen. The film is longer than one might first expect because of all of those powers of ten that must be counted to include all of the things astronomy covers.

#### A Scaled Model of the Solar System

Another way to give you a sense of the distances between things is to use a proportional (scaled'') model. In such a model, everything is reduced by the same amount, so all parts of the model relative to each other are of the same proportional size. (In the same way a good trail map you use for hiking or the road map you use for driving is a flat scaled model of the terrain you are moving over.) To create a scale model, divide all of the actual distances or sizes by the same scale factor (in the example below the scale factor is 8,431,254,000), so the scaled distance = (actual distance)/(scale factor).

For our scale model, let us use a yellow mini-basketball about 16.51 centimeters (6.5 inches) across to represent the Sun and then pace out how far the tiny planets would be in this scale model. Since the real Sun is 1,392,000 kilometers (865,000 miles) across, the scale model has all of the planets and distances reduced by an amount equal to (139,200,000,000 / 16.51) = 8,431,254,000 times. The largest planet, Jupiter, would be only 1.7 centimeters across (a dime) and about 92.3 meters away. Our little Earth (a grain of sand) would be closer: only'' 17.7 meters (about 18 big steps) away. Our Sun is much larger than the planets, and, yet, it is just a typical star! Here is a scaled model of our solar system:

Scaled Model of the Solar System
Object Real Diameter (km) Real Distance (million km) Scaled Size (cm) Scaled Distance (m)
Sun 1,392,000
16.51
Mercury 4880 57.910 0.058 (tiny! grain of sand) 6.9 (7 big steps)
Venus 12,104 108.16 0.14 (grain of sand) 12.8 (13 big steps)
Earth 12,742 149.6 0.15 (grain of sand) 17.7 (18 big steps)
Mars 6780 228.0 0.08 (almost 1 mm) 27.0 (27 big steps)
Jupiter 139,822 778.4 1.7 (a dime) 92.3 (92 big steps)
Saturn 116,464 1,427.0 1.4 (a button) 169.3 (169 big steps)
Uranus 50,724 2,869.6 0.6 (button snap) 340.4 (340 big steps)
Neptune 49,248 4,496.6 0.6 (button snap) 533.3 (533 big steps)
Pluto 2274 5,913.5 0.03 (small piece of dust) 701.4 (701 big steps)
Oort Cloud
11,200,000
1,328,400 (1,328 km)
Proxima Centauri 375,840 40,493,000 4.5 (handball) 4,802,700 (4,803 km)

I will usually use the metric system in this text. This system is used by every major country in the world except the United States. The United States will eventually adopt this system. Readers in the U.S. can multiply the kilometer numbers by 0.6 to get the number of miles and multiply the centimeter numbers by 0.4 to get the number of inches. Here is a picture of the planet orbits to help you visualize the vast scales of just the solar system.

The Oort Cloud is a huge spherical cloud of trillions of comets surrounding the Sun that is about 7.5 to 15 trillion kilometers across. In our scale model, the middle of the Oort Cloud would be about the distance between Los Angeles and Denver. Proxima Centauri is the closest star to us outside of the solar system (remember that the Sun is a star too!). Proxima Centauri would be from Los Angeles to beyond the tip of the state of Maine on this scale model (from Los Angeles to New Glasgow, Nova Scotia to be more precise!). In our fastest rocket ships (neglecting the Sun's gravity) it would take almost 70,000 years to reach Proxima Centauri!

Instead of using ridiculously small units like kilometers, astronomers use much larger distance units like an astronomical unit to describe distances between the planets and a light year to describe distances between the stars. An astronomical unit = the average distance between the Earth and the Sun, or about 149.6 million kilometers. For example, Jupiter is (778.4 million km)/(149.6 million km) = 5.203 astronomical units from the Sun. A light year is how far light will travel in one year. The distance D something travels in a given time interval t is found by multiplying the speed v by the time interval. In compact math notation this is: D = v×t. You can find out how many kilometers a light year is by multiplying the speed of light by a time interval of one year:

1 light year = (299,800 kilometers/second) × (31,560,000 seconds/year) = 9,461,000,000,000 kilometers (9.461 trillion kilometers---several tens of thousands of times larger than even the astronomical unit!).

The nearest star is about 4.3 light years away which means that it takes light 4.3 years to travel from Proxima Centauri to Earth. The rest of the stars are further away than that! The speed of light is the fastest speed possible for anything in the universe to travel despite what you may see in science fiction movies or books. It is because of the H-U-G-E distances and l-o-n-g times it would take extraterrestrial spacecraft to travel to the Earth that many astronomers are skeptical about extraterrestrial beings abducting humans.

The Sun is one star among over 200 billion stars gravitationally bound together to make the Milky Way Galaxy. A galaxy is a very large cluster of billions of stars held together by the force of their mutual gravity on each other. That definition is a loaded one that will be unpacked and examined in more detail in later chapters, but for now let us continue on our brief tour of the universe. The Milky Way is a flat galaxy shaped like a pancake with a bulge in the center. Stars and gas are clumped in spiral arms in the flat disk part of the Galaxy. Many stars are also found in between the spiral arms. Our solar system is in one of the spiral arms of the Milky Way and is about 27,000 light years from the center of the galaxy. The entire Milky Way is about 100,000 light years across. In our scaled model with the Sun 16.51 centimeters across, the Milky Way would be about 112 million kilometers across or about 38% of the size of the Earth's orbit around the Sun. Recall that Pluto's orbit is only 1.4 kilometers across on this scale---the Galaxy is MUCH larger than our solar system! Here is an artist's view of our galaxy with the Sun's position marked (note that our entire solar system would be smaller than the smallest dot visible in the picture!):

Let's reduce our scale model even more so that our galaxy is the size of the mini-basketball. The closest other galaxy is a small irregularly-shaped one about 13 centimeters away from the Sun toward the direction of the Milky Way's center. It is about the size of a cooked, fat breakfast sausage link in our scale model. Appropriately, the Milky Way is in the process of gobbling up this galaxy. Two famous satellite galaxies of the Milky Way called the Large Magellanic Cloud and Small Magellanic Cloud are about 30 centimeters and 35 centimeters away, respectively. The Large Magellanic Cloud is about the size of a tennis ball and the Small Magellanic Cloud is about the size of a ping pong ball. The Andromeda Galaxy is the closest large galaxy to the Milky Way: a ball 19 centimeters in diameter (a volleyball) about 4.8 meters away. The Milky Way and the Andromeda Galaxy are at either end of a group of about 30 galaxies gravitationally bound together in the Local Group. The Local Group can be roughly divided into two clumps with each clump having a large spiral in it: the Milky Way and the Andromeda Galaxy. Here are three views of the Local Group, each viewed from a position 90 degrees different from the rest. The Milky Way is the large dot at the intersection of the x,y,z axes and the Andromeda Galaxy is the other large dot.

The closest large cluster of galaxies is called the Virgo cluster (toward the direction of the Virgo constellation). The Virgo cluster has over 1000 galaxies in it and is roughly 50 meters away in our scale model. Notice that compared to their size, the galaxies are relatively close to one another. Stars inside a galaxy are relatively very far apart from one another compared to the sizes of the stars. You will see that the relative closeness of the galaxies to each other has a significant effect on the development of galaxies.

The Local Group and Virgo cluster are part of a larger long, narrow group called the Local SuperCluster, sometimes called the Virgo Supercluster since the Virgo cluster is close to the middle. The Local Group is close to one edge of the Local SuperCluster. In our scale model with the Milky Way the size of a mini-basketball, the Local Supercluster is about 190 meters long and the entire observable universe is about 49.5 kilometers in diameter.

The scale models described above are just a few of the many you will find on the internet. In the next section, a scale model for Time will be given. Before you go there, though, check out Nikon's "Universcale" that goes from the nanoworld to the universe in a very cool way (link will appear in a new window) or see Josh Worth's "If the Moon Were Only 1 Pixel" for a scale model of the solar system. Another page to check out for getting some perspective on our location and size in the universe is The Known Universe from the American Museum of Natural History (link appears in a new window and view the embedded YouTube six-minute video). Finally, Alex Gorosh & Wylie Overstreet posted a video of their construction of a true scale model of the solar system out to Neptune on a dry lake bed in Nevada.

## New discoveries

With the technological advancements over time, we were able to calculate something called transverse velocity, which indicates a colossal smash with our own galaxy in about 4 billion years. A team of researchers at the Space Telescope Science Institute were able to create a computer simulation depicting the process and the result of this collision. The results of these simulations were similar to those we observed in other galactic collisions, e.g. Antennae. Less than a 100 years after we proved it was, in fact, a galaxy, we were able to identify an exoplanet in the Andromeda galaxy.

There is still an ongoing debate around the Andromeda’s mass. Given the diameter which is more than twice as long as in the Milky Way, we have thought it was at least twice as massive as our galaxy. But in 2018, a group of researchers from Australia concluded that it might be as massive as the Milky Way, despite its size and number of stars.

In 1999 we observed a microlensing event, a type of cosmic event where excessive gravity bends the light and magnifies it to reveal otherwise obscure details. This allowed us to detect the first exoplanet in a distant galaxy that is approximately 6.34 times larger than our gas giant Jupiter.

## A Cosmic Microcosm

The Local Group exposes a challenge familiar to most scientists: Big objects are easier to find and examine than their more ubiquitous little cousins. Biologists can observe and study elephants more easily than they can microscopic creatures that outnumber them a millionfold. And stellar astronomers have little problem seeing massive, highly luminous stars across the galaxy, but they have to search deeply to discover smaller, fainter red dwarfs that make up 75 percent of the Milky Way.

Of the 100 brightest stars in the night sky, only five are also among the 100 closest. And no red dwarf shines brightly enough to be seen with the naked eye.

For galactic astronomers, the Local Group provides a near-perfect laboratory for exploring the smallest, most common galaxies. Our neighborhood collection comprises three spirals, two ellipticals, nine irregulars (including the LMC and SMC), and at least 40 dwarf elliptical, dwarf irregular, and dwarf spheroidal galaxies. To truly understand the universe at large, scientists need to study these abundant dwarf galaxies and how they relate to their scarce masters.

## How many stars are in the Milky Way?

By: The Editors of Sky & Telescope July 24, 2006 0

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What is the best educated guess for the number of stars in the Milky Way? I’ve seen figures ranging from “some 20 billion” to “just under 600 billion.” The first seems too low, whereas the latter too high.

The short answer is, nobody knows. But your reasoning is sound: 20 billion is too low, and 600 billion is likely — though not certainly — too high. According to University of Massachusetts astronomer Martin Weinberg, the Milky Way probably contains 200 billion stars, though that number “could be larger or smaller by a factor of 2 or 3.”

Astronomers have imaged only a small fraction of these stars. They guesstimate the total using a multi-pronged approach. First, they tally very nearby stars as a baseline. Then they observe other galaxies to see how starlight is distributed (most of the stars in our own galaxy are hidden behind clouds of dust). Finally, they apply the locally determined relationship between stellar numbers and light output galaxy-wide.

Each step is plagued by uncertainty. For example, the most common stars (late-type M dwarfs) are the least luminous and hence can be seen only nearby. Yet censuses are incomplete even in our backyard, says Georgia State University astronomer Todd Henry. In the last five years, Henry and other researchers have discovered 51 new stars within 33 light-years (10 parsecs) of our Sun, a 17 percent increase.

Astronomers have used telescopes as well as mathematic calculations to analyze the Milky Way galaxy. One of the clues about the shape of the galaxy happens when astronomers map the bright young stars along with the ionized hydrogen that’s in the disk of the Milky Way. They call these clouds the “HII regions” and they are caused by the ionization of the hot, new stars that have little or no protons and electrons.

Astronomers noted that this situation occurs in the arms of other spiral galaxies and helped to confirm the shape of our own Milky Way. Astronomers also measure the dominant colors and the quantity of dust in our galaxy to match them with other galaxies.

They have noted that the major arms of our galaxy seem to contain the most old and young stars and the minor arms seemed to be filled with the kind of gas and dust that is needed to make new stars. Our sun exists in a smaller, partial arm that is called the “Orion Arm or Orion Spur.” It, along with our solar system is positioned between the Perseus and Sagittarius arms.

The study of our Milky Way galaxy has allowed scientists to understand that the universe contains billions of other galaxies. However, other than our own Milky Way, there are only three other galaxies that we can see as fuzzy images without a telescope. When in Earth’s southern hemisphere, astronomers can see the Small and Large Magellanic Clouds which are considered to be satellite galaxies of the Milky Way.

Both of these are around 160,000 light-years away. The Andromeda galaxy is a bigger galaxy that can be viewed from the northern hemisphere of Earth, and it’s around 2.5 million light-years away. Andromeda is moving closer to our Milky Way galaxy and it’s thought that it will collide with us in around 4 billion years. To give you an idea of distance, it takes the light from our neighbor galaxies around 2.5 million years to get to Earth.

## Milky Way and Andromeda Collision

Surprisingly, our galaxy, Milky Way is expected to have a collision with another galaxy, called Andromeda. Even though the universe is expanding at every moment since its creation, the 2 galaxies, Milky Way and Andromeda are coming close to each other with an anticipated collision in about 4 billion years. Both galaxies are located close enough distance to overcome the expansion of the universe thus, their gravity is pulling each other resulting in this collision.

There are several consequences to this galactic collision. There are 2 types of collisions that can happen: One is stellar collision, which is that stars in each galaxy collide with each other and the other one is black hole collision, which is the collision of huge black holes in Andromeda and Milky Way. However, the probability for stellar collision to happen during this galactic collision is very low because the stars are distanced far enough to make it hardly susceptible to a stellar collision. In contrast with stellar collision, black hole collision is more likely to happen because the black holes in Milky Way and Andromeda have huge gravity force to pull not only each other, but also surrounding objects such as stars and planets.

Another surprising fact about the fate of our solar system during this collision is that due to rapid orbiting of galaxies in the process of the collision, our solar system might be affected and moved farther away from the center of the merged galaxy, or it might be cast out of the galaxy.

Even though the galaxies won’t collide in the imminent future, it is very interesting to look at the simulation of the galactic collision along with other aftermaths because it opens many possibilities that can happen in our galaxy. As a conclusion, we humans might need to start thinking of a way to prepare ourselves for this collision in the distant future.