4 Mind-Blowing Things About Stars

Oh come on.
You’re really gonna sit there and just focus on your life all day and not think about the universe at all?
Imagine if there were a bacterium in your fingernail who was smart enough to know a lot about the world—it had a pretty good sense of history, it had access to plenty of info about current events, and at night it could just turn its head upwards and look out at the world around it. But the bacterium decided instead to ignore all that almost all the time and instead obsess over whatever was going on in your fingernail.

We’d all be pretty unimpressed with that bacterium, wouldn’t we? And there you are, completely ignoring the fact that you live here—

—because you’re too busy putting on three different shirts because you can’t decide which one to wear, and people are getting drinks after work today so it’s important.
Since, as I’ve mentioned before, I’ve been seriously dating astronomy ever since I was three, one of Wait But Why’s missions will be to remind you not to constantly ignore the universe.

We’ll start by tackling an obvious topic today: stars.

When people think about the universe, they think about stars. Stars make up most of the visible matter in the universe, and despite all stars beyond our sun being unthinkably far away from us, we can see thousands of them with our naked eye at night.

For this post, I sifted through the endless fascinating facts about stars and picked out four that I find particularly mind-blowing—

1) There are an unfathomable number of stars in the observable universe.

So you know when you’re in a rural area at night and the moon isn’t visible and you can see a ton of stars?

In those optimal circumstances, you’re looking at around 2,500 stars at most. That’s about 1/100,000,000th of the total stars just in our own galaxy.

Speaking of which, take another look above at our gorgeous Milky Way. To help understand just how insanely big it is, here are a few facts:

– The diameter of the Milky Way is 100,000 light years. A light year is the distance light travels in a year. Considering light can travel around the Earth seven times in a second, a light year is a mind-bogglingly large distance. It would take our fastest spacecraft 18,000 years to travel one light year. And with the Milky Way, we’re talking about 100,000 of them.

– That also means that if you use a telescope to see a star on the far side of the galaxy, you’re actually seeing what the star looked like 100,000 years ago, since the light that left the star then is just reaching us now. Likewise, if at this moment, someone on the other side of the galaxy is checking out the Earth with a telescope, they’ll see a bunch of early humans and Neanderthals running around clubbing each other like fools.

– You might think that when you look closely at that above picture of the Milky Way, one of those little dots might be our sun. In fact, if you expanded the above photo to be the size of the Earth, you’d still need a microscope to be able to see our pinprick of a sun—if the Milky Way were the size of the Earth, the sun would be about 1/50th of a millimeter in diameter.

Massive. And all in all, the Milky Way contains between 100 and 400 billion stars.

And that’s just one galaxy.

In 1995, scientists picked out a little section of the night sky that was unusually devoid of stars. To the naked eye, and even in a normal telescope, this region looked empty and black. And the section was tiny—it covered the same amount of sky that a tennis ball would cover if it were 100 meters above you (and the image on the right shows the size of the region in comparison to the size of the moon in the sky at night).

The scientists used the Hubble Telescope to take a 10-day long exposure of the empty region to find out what was out there deep in the blackness. They came back with this:


To be clear, nothing in this photo is a star. Each thing you see—even the faintest little dot—is an entire galaxy. There are over 10,000 in this image, each one containing around 100 billion stars. And again, this is all in a pinpoint little square of the night sky.

Scientists used the info from this photo to postulate that the observable universe contains over 100 billion galaxies, which puts the total stars in the observable universe at somewhere between 10^22 and 10^24, or around 100 sextillion stars.

To put that in perspective, people at the University of Hawaii spent an unreasonable amount of time calculating an estimate for the number of grains of sand in the world—7.5 quintillion or 7.5 x 10^18.

That means that for every grain of sand on Earth, there are about 10,000 stars in the universe.


2) Stars are not close to one another.


Binary star systems aside, most stars are sitting there with no one around for huge distances in any direction, completely bored.

Our sun is no exception—the closest star to us, Proxima Centauri, is 4.24 light years away, or 70,000 years away in our fastest spacecraft.

So if the sun were a 4cm-diameter ping pong ball in New York, the closest star is another ping pong ball 1,153km (743mi) away in Atlanta.

3) Some stars are unbelievably huge.

The largest stars are called red hypergiants. One absurdly large one is called VY Canis Majoris. If you stacked 1,420 of our suns on top of each other, you’d have the diameter of VY Canis Majoris. Here’s what it looks like next to the sun:

Or, to bring back the ping pong ball-size sun, that would make VY Canis Majoris the height of a 16-story building. It would take an airplane about 1,100 years to fly around it, and if VY Canis Majoris were in the center of our Solar System where our sun is, it would swallow up everything out to the orbit of Saturn.

Another red hypergiant almost as large as VY Canis Majoris is Betelgeuse. You can see Betelgeuse on any starry night as Orion’s upper left shoulder—


4) Some stars are incredibly tiny and dense.


When a large star dies by exploding in a supernova, a gravitational collapse can take place and result in a neutron star.

Normal matter is made of atoms, and atoms are made up almost entirely of empty space. The only thing that gives an atom its mass is the tiny nucleus in the middle. To visualize this, picture an atom being a large sphere with a diameter of 1km—that sphere is so big you could stack two Empire State Buildings inside of it without hitting the ceiling. If that sphere is an atom, the nucleus would be a single pea floating in the middle, and the mass of the whole atom would be the same as the mass of the pea.

What happens during a gravitational collapse into a neutron star is that the atoms are squished so hard together that each atom’s empty space gives way and the peas in the middle of each atom are suddenly all bunched together. So now imagine that 1km-diameter sphere, except picture that it’s packed full with peas. So instead of that 1km space having a mass of 1 pea, it now has a mass of about 1,000,000,000,000,000 peas.

This is what happens when a neutron star is formed—there’s no empty space, just the neutrons that make up an atom’s nucleus packed together—and it’s a quadrillion times denser than a normal star.

The result is a tiny star with a diameter of only around 24km whose mass is as much as three suns or one million Earths. Below is the size of a neutron star—imagine packing 1,000,000 Earths into it:

Some amazing neutron star facts:

– A teaspoon of it has the same mass as 900 Great Pyramids of Giza.

The density of a neutron star is the same as compressing a Boeing 747 airplane into a small grain of sand.

– There are some that spin as fast as 642 times per second. That means that a dot on the star’s “equator” moves a greater distance than the Earth’s circumference every second.

– Neutron stars are hot. The inside of a neutron star is between 10^11 and 10^12 Kelvin—over 1,000 times hotter than the core of the sun.

Pretty overwhelming.

Okay, nice job. Now back to the fingernail.

Related Wait But Why Posts

How (and Why) SpaceX Will Colonize Mars

The Fermi Paradox

The AI Revolution: The Road to Superintelligence

  • This post took my breath away. Nicely done.

  • Anonymous

    Every word of that was jaw-dropping. Amazing!!

  • Love it! Space is an awesome thing to consider. Two things I would recommend just for any further interest…

    1. http://www.dailymotion.com/video/xa9raa_most-important-image-ever-taken-by_tech – a clip explaining a bit more about the Hubble Deep Field mentioned above. With a nice Pink Floyd soundtrack…

    2. The app Celestia, which lets you roam around space. I happily sit and forego 10 minutes of work to float around in the depths of space, read about the stars and planets. It’s good fun (if you’re into that sort of thing). I find it especially breath taking to go “Home” to the Sun from wherever I am in the universe, and fly up to it at the speed of light.

    But perhaps I’m just a nerd…

    Thanks for your work – this blog has become something I very much look forward to each week!

    • I love that video. And downloading that app as I write this.

    • Just hoping Celestia is not too big a banana for your Instant Gratification Monkey…

    • I don’t know how many nights I’ve been running Celestia and looked up to see it’s about half past go-to-bed o’clock. The monkey LOVES Celestia.

    • Anonymous

      HAHAHA excellent reference to the IGM…oops i think my Monkey just came out!

  • Andrea S.

    We don’t live on the Andromeda Galaxy 🙂

  • The most amazing part of the post is the revelation that I apparently live in the Andromeda Galaxy.

    • Ahhh good point. Fixed.

      Too bad, Andromeda is much prettier.

    • Anonymous

      Still great post, but we don’t live in that galaxy either, whatever it is. We don’t have any pictures of the Milky Way from outside, because, well, how would we take them? Easy enough to adjust the text to say a galaxy *like* the one pictured.

    • Anonymous

      No, it’s not. I like your blog but there are some inaccuracies here. This isn’t an artistic depiction but a computer-generated image carefully simulated, based on empirical data collected from astronomical observatories. That’s how scientists know how the Milky Way looks like. That’s not subjective art, but highly objective science.

    • Anonymous

      That’s not an artistic depiction, seriously. Did you even bother to research about it? This photos are computer-generated, based on observations from the earth.

  • Andrea S.

    Regardless, this is a great post! Can you please do one of these on black holes next?

    • Black holes are heavenly and upsetting. Yes, I’ll definitely get to them at some point.

  • Anonymous

    So now you’ve gone and parked a neutron star on Rhode Island. Why do you hate Rhode Island?

  • So, are you saying that we are an incredibly small part of the universe? What is the takeaway? Anyone?

    • Maybe that humans need to stop thinking the world revolves around them.

    • Anonymous

      Be careful what you wish for- you may find yourself thrown into the total perspective vortex.

  • Physics of Stars was almost certainly my favorite physics elective course – what makes everything you wrote more astonishing is how we figure these things out. For example, the ways we measure distances to stars (not to mention other galaxies) are really, really cool examples of problem solving. Because of optical effects like diffraction (d*sin(theta)=1.22*lambda for the interested) and more importantly, sheer distance, there are only a couple of stars in the sky that we can directly measure the size of – all others look like point sources of light. Literally everything else we know about the universe has to be inferred from indirect measurements (see the Hertzsprung-Russel diagram for a simple example on how we can derive how big stars are based on their effective temperature alone). Really, really cool stuff.

  • How is it possible that we have pictures of the milky way, which contains our solar system? we dont have a camera 100,000 light years away to take that selfie?

    Awesome post btw.

    • Easy answer: we don’t! We do have plenty of pictures of spiral galaxies similar enough to ours that everyone just uses images of them instead. That picture, after a quick reverse image search, appears to be some combination of illustration and an image of Messier 51 – but it’s close enough to reality that it doesn’t really matter.

    • How do we even know it’s close enough?

    • Well, even though we are inside the milky way, we can get a rough estimate of the shape of our galaxy by measuring lots and lots of stars and figuring out how far away they are (astronomers can also do this for clouds of gas and dust that make up the interstellar medium). After doing this enough (and some really fancy mathematical wizardry) we can make a rough map of our galaxy. Since we can observe thousands upon thousands of galaxies, we have a pretty good idea of what different types of galaxy look like in general. Using those two pieces of information, along with Photoshop, produces a fairly accurate map of the Milky Way.

    • Got it. Somehow this is like the bacteria looking around the finger and coming with a portrait of Angelina Jolie.

    • Except I doubt bacteria have access to radio telescopes:

    • True 🙂

  • back again, I am in a small town in the mountains of Colombia learning to Kite Surf. I was at the bus station going back to the city when your I got a message on my phone alerting me of your new blog post. I literally decided to stay, and miss the last bus home, because I had Internet connection where I was…haha, oh well, guess I get to kite surf again tomorrow, hehe…

    I now understand the meaning of quality content on a blog. I don’t think I’ve ever looked forward to reading a weekly article this much. Don’t self sabatoge 😉 your doing so awesome. looking forward to next weeks post


  • Dick

    Wildly fascinating.

  • Just… Just… spechless… … …
    This leaves me with more questions than answers.
    Thank you, keep on with this good stuff!!!!!!

  • Anonymous

    Thank you very much for your ongoing work.
    All very interesting, brightens my day, and provides some laughs for my colleagues in the office, each time I read your articles.

  • This is the kinkiest form of erotic humiliation. Your fetish for being humbled by the enormity of the universe.

    Nice read. I like how you take it upon yourself to shove people’s faces in the reality of their own insignificance. It’s a nice way to distract yourself from that of your own. Just as posting this comment is a nice way to distract myself from that of my own.

  • Anonymous

    This is wildy fascinating, I agree. Dumb question: How does Hubble photograph deep space? On Earth, you can’t photograph a house, say, that is located behind a mountain. Things overlap in 3D space from one persons perspective, preventing you from seing what is behind solid objects. How come this doesn’t work in space? How come, with all the solid galaxies in front of other solid galaxies, we can still see into deep space. No matter how far away things are, nothing is in front of anything preventing us from seeing it. What’s with that?

    • Space is really, really, really, really empty. There are things that block light in the universe though – light from distant stars is often attenuated by thin, dilute clouds of gas and the interstellar medium. Galaxies may look solid enough, but when two galaxies collide they basically just pass through each other without a single star touching another. Most of everything, to an astonishing degree, is “empty” space (zero-point energy notwithstanding).

    • How about the dark matter, dark energy and dark stuff?

    • All completely invisible! They don’t interact through the electromagnetic force (i.e. via photons) so we can only infer their existence through gravitational lensing and the acceleration of the expansion of the universe.

  • Insane numbers man!!! “NY Canis Majoris”… Just absurdly colossal!! How big could be the gravity field of that thing?!! I ve seen some documentaries about the universe and never heard about those awesome facts that u have picked for us, like “the teaspoon of 900 pyramids”… thank you so much!!
    I would like to know how to compress all my clothes in just one suitcase 😉 without falling in orbit around it

    • The magnitude of the gravitational potential is pretty easy to calculate: U(r)=-G*M/r
      And then the force is just -grad(U(r))=G*M/r^2=0.001203m/s^2 at the surface. This is actually much, much lower than the surface gravity of the Earth, because although VY Canoris Majoris is huge, it only weighs as much as 25 solar masses.

  • Anonymous

    Are we alone in the universe?

    HOW THE FUCK DID IT ALL HAPPEN? Okay, I can understand how Henry Ford built his empire and +/- how car manufacturing works, BUT HOW THE FUCK DID NATURE/UNIVERSE come up with these neutron stars, black holes and shit? Mind – BLOWN.

    Why do I want to poop? Okay I drunk coffee, but who we are to go to wars and create BS (=religion), when there is just SO MUCH TO EXPLORE?

    I’m totally lost 🙁

  • I’ve seen the video about the hubble before and was completely fascinated. I’ve been a huge lover of astronomy since I can remember. Excellent post. Looking forward to a blog on black holes. If not I’d settle for “why driving in the left lane under the speed limit makes you the biggest asshole on the planet.”

  • Anonymous

    “There are an unfathomable number of stars in the observable universe.” ….. There IS…. is….. ‘number’ is singular.

    • Anonymous

      “A big number of stars” = “a lot of stars”. “Lot” is also singular, but is still used with a plural verb: “there are a lot of stars”. It’s because the verb agrees with the word “stars”, not “number”. “A number of” can be omitted, and we will be left with “there _are_ stars”.

      If we were talking about “number”, it would go like this: “the number of stars is big”. Here the verb agrees with “number”: “the number _is_ big”.

      Only saying this because I’m reading this blog as part of my exam preparation 😀 In case I’m not the only one English learner here.

    • Anonymous

      *not the only English learner here 🙂 Now back to studying 😀

  • Anonymous

    Great article! More people need to know this stuff.

    One thing I’d mention, though. You said “You can see Betelgeuse on any starry night as Orion’s upper left shoulder”. Technically you can only see it in the night sky during mostly the winter months, with January being the best time to view it. During summer it’s up during the day, so it’s drowned out by all that blue sky.

    • *it’s best to see it around January in the evening sky. You can see it in the pre-dawn sky as early as August 🙂 Just a minor correction. Most people don’t realize just how much the sky “moves” over the course of a night. You can observe in the evening and then wake up pre-dawn and observe a whole different set of objects… or you can just stay up the whole night 😀

  • Thanks for posting this. I’ve also been involved with astronomy since a young age and am now finishing building a 16″ f4.5 scope. Can’t wait to do my part and share the universe with anyone and everyone through the eyepiece 🙂

  • Anonymous

    How scientists know about all density and temperature of stars? Its just speculation if u ask me

  • Anonymous
  • This has nothing to do with this post. But have you ever taken the Myers-Briggs personality test? It’s rather fascinating and I’ve spent hours procrastinating on… but I digress. Are you an INTP?

    • I actually have been told I’m an INTP.

      Creepily well done…

    • Anonymous

      Lol… I was going to ask you the same question. No, actually, I was going to announce that you are indeed an INTP 😀 No need to take the test now =))

  • Anonymous

    This is absolutely amazing. And I loooove how you ended this post. Indeed back to the fingernail!

  • Estimated time for a photon to bounce around inside the Sun before bursting from its surface as a little piece of sunlight: between 10,000 and 170,000 years.

  • Anonymous

    Very nicely explained. I bet you loved the Powers of Ten video back in the day! It maybe had more visual impact before the advent of Google Earth, but still one of my favorite ways to imagine the size of the galaxy.

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  • Alberto

    Here is a beautiful website showing the scale of universe: http://htwins.net/scale2/

    Have fun!

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  • KylieCone

    I have to do a stupid science project model of stars and galaxies and they want a 3-5 min speech!!!!!

    • Mike


      stupid? 🙁

      you make me sad.

  • Paul Monfils

    Thank you for this captivating article.

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  • RuthAMontgomery

    ….Some time hit the waitbutwhy Find Here

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