## how many stars are there? -- 10/14/20

Today's selection from Space at the Speed of Light by Dr. Rebecca Smethurst. How many stars are there in the universe?

"It is nigh on physically impossible to count the number of galaxies in the universe because a) there are so many and b) how can we be sure we've found them all?

"But, to put some sort of lower limit on this, we can use an image that the famous Hubble Space Telescope (HST) took about a decade ago. Astronomers decided to use HST to stare at the darkest patch of sky that we know, in the con­stellation Fornax, in the Southern Hemisphere night sky, to see what they could find. They took an image that was a 2 x 2 arcminute square patch of the sky. An arcminute is a funny unit -- it is a sixtieth (1/60) of a degree, and an arc­second is a sixtieth (1/60) of an arcminute. Given that the whole sky is 360 degrees around, it's a very small patch. It's an image that is 5 percent of the size of the Full Moon in the sky. Astronomers didn't really know what to expect to find in this tiny dark patch of sky, but the latest count on the number of stars found in the image of that patch is four. And the latest count of the number of galaxies is about five thousand -- everything from beautiful nearby spiral galax­ies to distant galaxies that we detect as just a single pixel.

 Hubble eXtreme Deep Field (HXDF) taken in 2012

"If we take that number and apply it to the rest of the area of the sky, we can estimate that there are at least 100 billion galaxies in the universe. Remember, this image was taken of the darkest patch of the sky, so in other regions we should see even more galaxies (plus all the galaxies that are still too faint for us to see). It's more likely that there are another couple of zeros on the end of that number. So, let's make our estimate a round trillion galaxies. Not only that, let's say that each galaxy contains, roughly, 100 billion stars. So, perhaps, we can estimate that there are at least 100 sextillion stars. That's 100,000,000,000,000,000,000,000 stars in the universe. So, if one in a quintillion stars might develop life, and there are at least a hundred sextillion stars in the universe, then perhaps there are a hundred thousand planets out there in the vastness of space that might have the right conditions to develop intelligent life!

"People often ask me, how -- as an astrophysicist know­ing and thinking about these numbers all the time -- I don't get overwhelmed by it all. How do I stare at the sky without being entirely crippled by anxiety at the sheer scale of the whole thing and our own insignificance? Firstly, in day-to­-day life -- whether you're at your desk crunching data, in an office, at home, or on a train -- there isn't time to stop and think about it. But, when I look at the majesty of the night sky, with the Milky Way stretching out overhead in a huge arc of stars, I don't feel anxious. I feel limitless. Like there are infinite possibilities out there and I could be part of any one of them. The scale doesn't scare me; it thrills me. Like the protagonist at the beginning of a good adventure novel yearning to see the world and get out of their small town. When I look up at the sky and think about the sheer number of stars out there, I can't help but get excited about drawing the conclusion that we can't be the only planet whose cards came up right in the game of life."

#### author:

Dr. Rebecca Smethurst

#### title:

Space at the Speed of Light

Ten Speed Press