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How Come I See Fewer Stars Than I Remember As A Child?

I’m past eighty years old. I remember as a child the stars looking like they were almost on top of us, so close and so bright and a billion of them to boot. Now they look so far away and almost faint. No it is not my eyesight. Have the stars moved away from us to that extent in just 80 years and if so how long before they are not seen at all.
The constellation Orion, imaged at left from dark skies, and at right from Orem, UT. Orem, UT is hardly a large city. This is intended to highlight the fact that light pollution is a problem everywhere, not just in cities with tens of millions of inhabitants. Image credit: Flickr user jpstanley, CC BY 2.0.

The constellation Orion, imaged at left from dark skies, and at right from Orem, UT. Orem, UT is hardly a large city. This is intended to highlight the fact that light pollution is a problem everywhere, not just in cities with tens of millions of inhabitants. Image credit: Flickr user jpstanley, CC BY 2.0.

Originally posted at Forbes!

Unfortunately, the stars haven’t moved, and I believe you that it’s not your eyesight either, because there’s another known and astronomically obnoxious thing that’s happened over the past decades. The amount of light pollution in the night skies has increased dramatically in the past few decades, which is largely because there are a lot more lights, illuminating our cities with no particular care for the darkness of the sky.

The increase in lighting is generally a good thing – well lit streets make people feel safer walking at night, for instance. However, the typical streetlight sends a lot of light up into the night sky, and not just down onto the sidewalks and streets. We also do a lot more lighting up of entire buildings now than we used to, several decades ago, and that light reflects off the building into the night sky. With the advent of cheap, bright LEDs, many of these lights are getting even brighter.

Marina Bay Sands, Singapore, at night. Singapore’s light pollution is so severe that the entire population will never use their night vision. Image credit: Leonid iaitskyi, CC A-SA 3.0.

Marina Bay Sands, Singapore, at night. Singapore’s light pollution is so severe that the entire population will never use their night vision. Image credit: Leonid iaitskyi, CC A-SA 3.0.

The combination of all the streetlights, spotlights, and building lights work together to fade our night sky. The more light pollution there is, the less black the night appears – the sky will only darken to a gray – and the more light pollution, the fewer stars are visible.  The faintest stars fade out rapidly, and relatively soon, there are only a few stars which remain.  

If you make it out to a truly dark spot, the stars of your childhood are still there – literal thousands of them, filling the night sky. But many people who live in cities have no way to get out to see the night sky in a remote area, so the dark, glittering sky is either a distant memory or feels like an experience they will never get to grasp.  This isn’t uncommon. It’s now estimated that 30% of the world’s population cannot see the Milky Way from their homes. This goes up to 80% if you live in North America.

We could make some improvements – the loss of the Milky Way doesn’t have to be permanent. We could choose to not illuminate the sky quite so much while we continue to light the streets for safety reasons.  Shielding the light so it can really only shine down would be an easy first step.  Switching off the lights that light up an entire building – even for a few hours in the middle of the night – would help any would-be meteor shower watchers getting up to watch the sky.  Any improvements we can make will bring the night sky back to everyone.  In the mean time, we’ll have to push for more lighting improvements, so that the only dark skies aren’t just in remote wildernesses.  If you’re ever in a dark area, far from a city, remember to look up.

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How many stars live in our galaxy?

This is actually a surprisingly tricky question to answer. The current wisdom suggests we have somewhere between 100 and 400 billion stars in our galaxy. You’ll notice that this is not very precise. 300 billion stars is a pretty sizable number of stars to be able to throw around, even by astronomical standards, which are notoriously wobbly. Why is this measurement so hard?

Fundamentally, it comes down to the simple fact that we’re inside the thing we’re trying to measure, and that the galaxy is a very complicated place.

We can’t just go out into the sky and count the number of stars we see, add them all up, and get the right answer. As a first problem, that would take way longer than anyone is willing to spend counting stars. If you were to spend one second counting each star, and we’re right that there are at least 100 billion stars to count, you’d have at least ~3,180 years worth of work laid out in front of you. On top of that, the galaxy is full of a bunch of pesky gas and dust, which blocks out light from stars behind it. The gas and dust tends to be most dense within the disk of our Milky Way, which is exactly where we’re sitting. Any time we look along the disk of the Milky Way, our view is heavily obscured, so we’d be missing a large fraction of the stars just because their light never reaches us.

So other than counting all the stars, what can we do? There are a couple methods left. A very crude way of counting is to take very detailed images of a few (hopefully low dust and low gas) patches of the sky - by proxy this is looking at a few patches of the Milky Way - and try to statistically piece the galaxy together. Look away from the disk, and how many stars do you see? How much area in space should look like that region? Multiply your density of stars by the area, and you’ll get out a number of stars in that area. Do the same thing again for a region of the galaxy near us, and near the center of the galaxy, and you can - approximately - work the number out, or at least get a sense of how big the number is likely to be.

You can also take the total amount of light that is given off by the galaxy and try to work backwards to figure out how many stars you need to have to create that much light. We will again have the problem of gas and dust blocking out some of the light, but certain wavelengths of light allow us to see through gas and dust to some extent, and we can use information from those other wavelengths to try and correct for the dust. The bigger problem at this stage is that working from total light to a number of stars, you need to know how many of each size star you expect to make. If your galaxy is good at forming very big stars, these stars are also extremely bright, so you need fewer stars to produce the same amount of light than if the galaxy is good at making very small stars, which tend to be dim. You can add quite a few very small dim stars into the galaxy before you change the amount of light produced by the galaxy by a significant amount - which is part of why the total number of stars in the galaxy that we calculate out could vary by so much. Galaxies produce stars of all sizes, but the exact ratio of small to large stars is still something astrophysicists are trying to understand - or tell if indeed it is a single number. We use the stars very near to us to get a general idea of what the distribution is like, but the small faint stars are (unsurprisingly) hard to spot, so this is still a difficult thing to look for.

Given how hard these measurements are to make, it’s perhaps less surprising that we have 300 billion stars worth of wiggle room in our numbers. Whether the answer be 100 billion or 400 billion, this is still an astounding number of stars, and given that we’re finding planets around almost every star, an even more astounding number of planets.

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