# A star or a galaxy?

When we look into the beautiful sky in the night, exclaiming how beautiful these shining stars are. My question is how could we tell, whether any of these shining "point" is a star or a galaxy?

If indeed many of these are shining galaxies, then what roughly the percentage is of these shining galaxies in the sky (using the naked eye only without telescope)? (And why.)

• Does "by using a telescope" count as an answer? Or are you considering naked eye observations only? Jan 2, 2020 at 3:15
• Very few galaxies can be seen with the naked eye although there are some (Andromeda, for example).
– Tom
Jan 2, 2020 at 19:30
• Here is an excellent video that shows how visible galaxies are when looking out from any arbitrary point in space: youtube.com/… Space is REALLY big. Jan 2, 2020 at 23:24
• If you did some research you could find that only the andromeda galaxy is visible with the bare eye, as a faint blob. Jan 3, 2020 at 0:47
• Jan 3, 2020 at 2:32

With the naked eye, virtually every point you see is a star.* That's because there are very few galaxies that are visible with the naked eye.

With telescopes, for many galaxies you'll be able to resolve multiple light sources (aka. stars). That lets you tell it's a galaxy. For those galaxies that are further away, you can still tell by taking a spectrum. Galaxies have fundamentally different spectra from stars, because they're composed of lots of different stars at different metallicities, temperature, etc (+ other stuff).

The percentage of stars/galaxies you see depends on what you're using to observe. In the Hubble Deep Field for example, virtually every point is a galaxy. Comparatively, if you're using an ordinary pair of binoculars, virtually every point is a star.

*Some of the brightest points will not be stars, but planets.

• And the few galaxies that are visible to the naked eye, like Andromeda, are diffuse patches rather than points. Jan 2, 2020 at 17:50
• Hmm. You might want to mention planets in the first sentence. Venus is by far the brightest point in the night sky, now. Jan 2, 2020 at 19:36
• @jamesqf: …but there are also several prominent more or less diffuse lights in the night sky that are not galaxies, but rather nebulae (like the Orion Nebula, whose center is easily mistaken for a star if you don't notice the fuzziness) or star clusters (like the Pleiades, which can appear more or less nebulous depending on your eyesight and viewing conditions), etc. So a diffuse appearance is a necessary criterion for something being a galaxy, but not a sufficient one. Jan 2, 2020 at 22:30
• @EricDuminil are they? There are several thousand stars that can be seen with the naked eye and only 8 planets, so "virtually every point" sounds correct to me. Jan 3, 2020 at 9:55
• @Allure: Sadly, due to light pollution, it's often possible to only see a few hundred stars at most. And of those points, the brightest ones will be Venus/Jupiter/Saturn/Mars. Or the ISS or an Iridium flare if you're lucky. There are very few days during which it's not possible to see at least one of the bright planets. Jan 3, 2020 at 10:24

Most such objects are too faint to see with the naked eye. There are about 9000 visible stars. They are nearby stars in our galaxy, which contains about a quarter trillion stars.

The nearest big galaxy is Andromeda. It is bright enough to see at least part of it. It appears as a fuzzy patch, not a point. It is 200,000 light years across and 2.5 million light years away. That means it is about 3 degrees across if you count all of it, not just the visible part. That is bigger than the sun, though much fainter.

On the scale of the galaxy, stars are tiny and separated by large distances. Our sun, bigger than a typical star, is 864,340 miles across. The nearest star is 4 light years away, or 23 trillion miles. If the nearest star was like the Sun. size/separation would be about $$4 \cdot 10^{-6}$$, a very tiny dot indeed.

Of course there are galaxies so far away they appear as points. But they are all too far away to see with just your eye.

This shows relative apparent sizes of various objects. https://xkcd.com/1276/

• It's a bit euphemistic to say that the sun is a "typical" star when the vast majority of stars in the Milky Way is red dwarfs... Jan 2, 2020 at 18:41
• I should have looked up the numbers. I fixed some. Thank you for the corrections. Jan 4, 2020 at 4:48

The Andromeda galaxy is the closest galaxy we can see with the naked eye A hardly visible and faint blob (you can see it the best by looking a bit beside of it, but I'm not sure where. I've seen it a few times but can't remember where. In any case on the Northern hemisphere). See here. Maybe this doesn't answer your question completely, but I've tried. Other galaxies can be observed with the naked eye as well (depending on their power and distance). See the link below. It's kind of silly to say that the nearest one we can see is the Milky Way. The form of which has relatively been discovered quite recently. When you're inside of something it's much more difficult to observe its form than when you are ouside of it.

There it is, seen through a telescope. Sometimes it's called a Nebulae, which in fact it isn't.

• The Magellanic clouds (and of course also the Milky Way) are galaxies that are visible with the naked eye. Jan 3, 2020 at 1:43
• The Magellanic Clouds are just little satellites to our Milky Way. To say the Milky Way itself is visible, well...Oh, it was me who voted you down. Jan 3, 2020 at 2:23
• I've never seen the Andromeda galaxy, but I've seen the Magellanic Clouds countless times. ;) Wikipedia lists several more naked eye galaxies: en.wikipedia.org/wiki/List_of_galaxies#Naked-eye_galaxies Jan 3, 2020 at 2:35
• Well, I have. physics.ucla.edu/~huffman/m31.html Jan 3, 2020 at 2:50
• "There it is, seen through a telescope." To be precise, you would need to change "seen" to "photographed" or "imaged". If you tried to see it through the same telescope that took the photograph, you would be very disappointed. Jan 9, 2020 at 22:21

Visibility depends on power per solid angle. Since for any galaxy, the high power locations like the starts only occupy very limited volumn then very limited solid angle in the solid angle of the galaxy, the everage power per solid angle per galaxy will be far more lower than a star. This is why commonly you can't see galaxy with human eye. Even some star in the galaxy is so bright that make the power per solid angle of the galaxy is visible to human eye, what you really see is that bright start instead of the galaxy containing it.