Starts With A Bang!

Whose Stars Are These?

When you look up at the night sky, one of the first things you learn is that every point of light you see is either a star or a planet:

But what happens if you look through a pair of binoculars? Well, suddenly, you can see a lot more stars than you used to. Because it focuses light coming from a larger region of the sky into your eyes, it lets you see fainter objects. Instead of just about 3,000 stars, binoculars let you see over 100,000 stars! A part of the night sky would look like this instead:

But our technology doesn’t stop at binoculars; we’ve build telescopes, both large and small, that have incredible amounts of light-gathering power! The unaided human eye is about 100 times less powerful than a pair of binoculars, and a small telescope (about 6-8″) is another 100 times more powerful than binoculars are. When you start to use small telescopes, you can see details that are impossible with simple binoculars. Take a look (and click here for the full image):

This is just a picture taken by an amateur astronomer of a comet (the green thing, Tuttle 8P) that happens to be passing by a nearby galaxy (M33, the diffuse thing). And we can see that not everything is a star after all.

But our reader Greg is very savvy, and knows what really powerful telescopes can see.
He asks the following:

Approximately how many of the stars in a picture such as this are in our Milky Way galaxy, and how many are outside our galaxy just floating in space? Is it possible to see individual stars in other galaxies such as Andromeda?

In reality, the largest and most powerful telescopes can see objects that are anywhere between 10,000 and 1,000,000 times fainter than what a small, amateur telescope can see. Let’s take a look at Greg’s picture:

Now there are a bunch of stars in that image, but how do we know which ones belong to our galaxy and which ones belong to Andromeda, which is the closest large galaxy to us? This is a question with an easy answer: they’re all ours! While we can resolve individual stars sometimes in Andromeda, such as Cepheid variable stars, they don’t show up in a normal telescope image like this. How do we know? Let’s take a very similar galaxy to both ourselves and Andromeda, but the only difference is that it’s about 6 times farther away. This is the Sculptor Galaxy:

Notice how it looks very similar to the Andromeda Galaxy, except with far fewer star-like speckles? That’s because all the stars are here, in our own galaxy!

Want to take an extreme case? The Hubble Deep Field and Hubble Ultra-Deep Field images were made by pointing the telescope at a patch of sky known for being completely dark; having no known stars in them. You just sit there and point your telescope there, overexposing the image, and finding what turns up. The results look like this:

But the amazing thing is that every speck of light there is a galaxy; there are no stars in this image. So if we want, we can zoom in on a large, bright galaxy as best we can and see what it looks like:

Notice the absence of individual stars? It’s because they’re all in our galaxy. In fact, if we take that ultra-powerful Hubble Space Telescope and point it, say, at the sculptor galaxy above, we can see some individual stars. But they’re so faint and so miniscule compared to the stars in our galaxy that they get dwarfed. Take a look at the image below: note the foreground star in our galaxy (halfway down, about 1/5 of the way from the left border) and all the other stars, which belong to NGC 253 (sculptor’s scientific name):

See what I mean? They’re all our stars. Looking out at the Universe means looking out through our galaxy, and unbelievably, even in outer space, our own neighborhood gets in the way!

Good question, Greg, and I hope I answered it well for you!