Stars and the Milky Way. How many stars can one see with the naked eye?

How far away can you see with the naked eye when you look at the sky?

An interesting question: if you look up in the Earth’s sky with the naked eye, assuming you have healthy vision, how far away can you see? Could you see beyond the Milky Way galaxy, for example?

The Andromeda Galaxy: The Farthest Object Visible to the Naked Eye

Andromeda Galaxy is the farthest object in the sky that we can see with the naked eye. It is approximately 2.5 million light-years (770 kiloparsecs) from Earth and the nearest large galaxy to the Milky Way.

So, you can see 2.5 million light-years away if you look in the sky with the naked eye.

2.5 million light-years distance is equivalent to about 23.65 billion trillion kilometers (2.36×1022 km) or 14.7 billion trillion miles (1.47×1022 mi).

1 billion trillions = 1 sextillion

So a person with a healthy vision can see 14.7 sextillion miles or 23.65 sextillion kilometers in the sky.

If Andromeda Galaxy were brighter, this is what we’d see

If you don’t know exactly where to look, spotting the Andromeda Galaxy can be quite challenging. Imagine if it were significantly brighter; its presence in the night sky would be nothing short of spectacular. To put it into perspective, consider the moon as a reference. If our neighboring galaxy shone as brightly, the view would be truly awe-inspiring, as illustrated in the image below:

How far away can you see with the naked eye when you look at the sky? Andromeda Galaxy is the farthest object we can see in the sky with the naked eye. If Andromeda Galaxy was brighter, this is how it would look in the sky.
As the farthest object visible to the naked eye, the Andromeda Galaxy can be elusive if you don’t know where to look. The Moon covers about 0.5 degrees in the sky, while the Andromeda Galaxy spans several times that size. So, if it were brighter, its appearance in the sky would be spectacular.

The Andromeda Galaxy, also known as Messier 31, M31, or NGC 224, a mere 2.5 million light-years distant, is the closest large spiral galaxy to our own Milky Way. Andromeda is larger than our galaxy and is visible to the unaided eye as a small, faint, fuzzy patch, but because its surface brightness is so low, casual skygazers can’t appreciate the galaxy’s impressive extent in planet Earth’s sky.

2006 observations by the Spitzer Space Telescope revealed that Andromeda contains one trillion (1012) stars – at least twice the number of stars in the Milky Way, which is estimated to be between 200 and 400 billion. The mass of the Andromeda Galaxy is estimated to be 1.5×1012 solar masses, while the Milky Way is estimated to be 8.5×1011 solar masses.

Andromeda galaxy received its name from the area of the sky in which it appears, the constellation of Andromeda, which was named after the mythological princess, the daughter of Cepheus and Cassiopeia, king and queen of ancient Ethiopia in Greek mythology.

Andromeda Galaxy is on a collision course with the Milky Way

In the far, far future, humans (assuming humans still exist, of course) will see the Andromeda Galaxy much more clearly and more spectacularly. This is because the Andromeda Galaxy is on a direct collision course with our galaxy, the Milky Way. It is expected that the Andromeda Galaxy will collide with the Milky Way in about 4 billion years. Here’s how the Earth’s sky might look shortly before they begin to collide:

Andromeda Galaxy (M31) is on a collision course with the Milky Way. This illustration depicts how the sky might appear from a distant future world as the central parts of these galaxies begin to collide and interact.
Recent Hubble Space Telescope observations suggest that the Andromeda Galaxy (M31) is on a collision course with the Milky Way. Although there is a margin of error in the sideways velocity measurements, their outer halos are likely to become gravitationally entangled. This will eventually lead to the two galaxies merging into a single large elliptical galaxy. This illustration depicts how the sky might appear from a distant future world as the central parts of these galaxies begin to collide and interact. Illustration Credit: NASA, ESA, Z. Levay and R. van der Marel (STScI), and A. Mellinger.

Recent Hubble Space Telescope images have allowed astronomers to carefully plot the slight displacements of stars in the Andromeda Galaxy (M31) relative to background galaxies. These observations suggest that the center of M31 is on a direct collision course with the center of our Milky Way galaxy. However, there is still a significant margin of error in the sideways velocity measurements, leaving room for the possibility that the central parts of the two galaxies might narrowly miss each other.

Even so, their outer halos are likely to become gravitationally entangled. Once this happens, the two galaxies will become bound, engage in a cosmic dance, and eventually merge into a single large elliptical galaxy over the course of several billion years. The image above is an artist’s illustration of how the sky might look from a distant future world when the central parts of each galaxy begin to collide and interact destructively.

A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy or possibly a large disc galaxy. Such mergers are common among galaxies in galaxy groups. The fate of the Earth and the Solar System during this event is currently unknown. Before the two galaxies merge, there is a small chance that the Solar System could be ejected from the Milky Way or even become part of the Andromeda Galaxy.

When will Andromeda and Milky Way collide?

According to a 2019 study published in The Astrophysical Journal, the collision between our Milky Way and fellow spiral galaxy Andromeda will occur about 4.5 billion years from now. The new research is based on observations made by the European Space Agency’s (ESA) Gaia spacecraft.

How to find Andromeda Galaxy in the sky?

The Andromeda Galaxy is best seen during autumn nights in the Northern Hemisphere. First, you have to find a place without light pollution, far from the city lights on a dark, moonless night. Evening is an ideal time. Then, look to the east. You’ll need to find the “Great Square of Pegasus”, a large square pattern of stars.

The four stars of the square are a conspicuous sight in the autumn night sky. Then, follow the stars from the top left corner of the square towards the star Mirach (Beta Andromedae). Then, jump to the star above Mirach – it is Mu Andromedae (μ Andromedae). Andromeda galaxy is very close to it in the sky, approximately at one o’clock direction.

This image can help you to find the Andromeda Galaxy in the sky
This image can help you to find the Andromeda Galaxy in the sky

Sources

M. Özgür Nevres

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