Astronomy questions from sixth graders, Part 4

By Phil Plait | May 22, 2008 9:32 am

Note: There is a special note to teachers at the bottom of this post. If you’re a teacher, please be sure to check it out!

My friend Tina is a teacher at the Saegert Sixth Grade Center in Austin, Texas. She asked her sixth grade students to send me questions they had about astronomy, and I answer them on camera. There were so many I had to split this into five parts! I’ll be posting one part every day, first thing in the morning. To catch you up, check out Part 1, Part 2, and Part 3. Below is Part 4.

The questions asked in Part 4 are:

1) Is there another planet past Pluto or inside Mercury’s orbit?

2) Why can’t we live without the Sun?

3) How do stars form?

4) Have the constellations changed?

5) Why is space black? Why doesn’t the Sun light it up?

6) Do galaxies move around in space and do they collide?

7) What is the biggest galaxy?

SPECIAL NOTE TO TEACHERS: Many schools block access to YouTube. There is another video hosting platform called TeacherTube, which is designed to be used in schools. I’ve uploaded this video (Part 4) to my channel there, where you can access it in your school (note: the video is in higher-resolution on YouTube). If you do, please let me know! I’d love to know what the students thought of the video — warts and all.

CATEGORIZED UNDER: Astronomy, Cool stuff, Science, Video Blog

Comments (22)

Links to this Post

  1. Science For the Kids!! « THE SKEPTIC DAD | May 30, 2008
  1. Dumb Guy

    Those are some fantastic questions! And, being a dumb guy, I would like to know some of those answers myself!

  2. Todd W.

    The galaxy collision question made me think of another one. If, by some freak chance, a large proportion (say, 60%) of the matter in two galaxies collided, rather than just passing by each other and letting the gravity do the work, what would happen?

    I know the probability of that happening is next to 0, but what if?

  3. Mighty Favog

    I’m not sure you answered the question about why space is black well enough to be understood by people who don’t understand this. A lot of people have difficulty with the concept of “nothing” as well as understanding the role of light in vision. I think it is also common for people to think of space as a substance. Consider the following conversation I had with my mother, a college educated woman who is otherwise rather sharp on everyday common sense. We were watching Star Trek and she began with the following comment:

    Mom: “That’s not right. There’s no color in space.”

    Me: “Why do you think there’s no color in space?”

    Mom: “Because there’s no light.”

    Me: “Why do you think there’s no light in space?”

    Mom: “Because space is black.”

    At this point my mind was boggling with how to approach this. So I started with the basics:

    Me: “Light has to bounce off of something in order to come back to your eye so it can form a picture of it. Space appears black because you’re looking into an area where there is nothing to see.”

    Mom: “That’s ridiculous. How can light make a picture?”

    This was followed by an exchange that established that she also thought of space as a sort of black substance that was unique and different from the space we inhabit on earth. A couple of other people I know have exhibited similar misunderstandings. These views may appear rather childish to you and me, but from what I see from time to time on TV, I think a lot of people have such ideas. Consider the laughably child-like view of space and astronomy on the old show Space: Above and Beyond. I still shudder when I think of that show.

    It seems clear to me that the child who asked the question thinks of space as a substance and probably doesn’t understand the role of light in vision as well, and so may not understand your explaination.

  4. Wait, brown dwarfs are large enough to sustain hydrogen fusion? I thought they were only heated by gravitational contraction and a bit of deuterium fusion here and there.

  5. BILL7718

    Bah. My wife is a 5th grade teacher and has her class send me questions (I’m a software engineer with geeky friends to help). My work blocks teachertube, too. =( Hopefully she can see it from school.

  6. I’m seriously hoping there are some young minds being blown by watching your answers. You’ve got a great way of explaining astronomy. Maybe this will spawn some young space geeks.

  7. Thanny

    Brown dwarfs do *not* fuse hydrogen. They range in size from about 13 times to 80 times the mass of Jupiter. Higher than 80, and it starts fusing hydrogen, and becomes a red dwarf, which is the actual smallest type of star.

    I very much doubt the black sky question was about Olbers’ Paradox. Rather, it was about why the sky is bright blue during the day time, and black at night. The correct answer is both simple and complex. Simple, in that you could merely say that there’s no air in space to scatter photons. Complex, in that you’d have to explain the scattering process, and why it isn’t relevant on the night side of Earth, where there’s still plenty of air.

  8. Good video, like the other ones :)

    But the link named “Part 4”, does actually link to Part 3 on YouTube (I like to see this in good quality, so I always go there). That’ll have to be fixed.

    Looking forward to part 5!

  9. zeb

    I hate to suggest this so far into the series, but it would cool if there were diagrams to go along with your speech. Not that your hand gestures are inadequate, but, that, um…I guess I thought I’d have an ending to that sentence when I got to it. Oh well!

  10. Redx

    I’ve always kinda wondered about IO, which gets an awful lot of heat from tidal effects of Jupiter and its moons. We know xenophiles can create ecosystems without sunlight.

    Of course, we couldn’t live there, but I find the idea of live based on tidal energy kinda neat, and despite being pretty unlikely, I don’t see why it wouldn’t be possible.

    The energy is coming from somewhere of course, which means eventually all those moons are probably going to end as part of Jupiter, but on what time frame?

    Once the sun goes giant, we’re cooked, but IO, I believe, is far enough off that it shouldn’t be consumed. If there is enough energy in Jupiter’s moons to outlast the sun, then you can have an operational ecosystem orbiting a dark sun, which I think is cool(if astronomically improbably, of course).

  11. Greg in Austin

    Are there transcripts of these videos? I cannot watch any streaming media while at work. :(

    I am assuming that none of the answers are: “Because God did it!”


  12. D’oh! Link corrected.

    I was going to insert images and such, but that is a MAJOR pain using iMovie, and with 5 videos would have taken me many many hours. So I left them as is.

  13. Sili

    I have to say that these kids are asking really good questions.

    Nothing ‘Young Earth’ or geocentric either.

  14. Tom Marking

    “in that you’d have to explain the scattering process, and why it isn’t relevant on the night side of Earth, where there’s still plenty of air.”

    It is relevant at night time. It’s the explanation for why stars twinkle. As far as Olbers paradox goes, if you had eyes that could see at millimeter wavelengths the entire sky would be more or less uniformly bright in all directions. Hence at certain wavelengths there is no paradox. It only appears to be a paradox at visible wavelengths. Thus the whole explanation about infinite space and infinite time is a bit off.

  15. StevoR

    Like Greg in Austin I’d really love it if The Bad Astronomer could provide transcripts – y’know these could almost make for a book in themslves! 😉

    (In fact, Fred Watson from the Anglo-Australian telscope up at Mt Stromlo has already written something a bit like that but based on a radio show – ‘Why is Uranus Upside down?’ (Publishers : Allen & Unwin, 2007 ) which I’d like to recomend if Phil Plait can bear me recomending astronomy books other than his here! 😉 )

    Since I need the practice and for the sake of having fun with this I’ve got my own attempted answers to these questions below :

    (& also on the other parts via the links too …)

    Mind you although I can’t hear or see them myself I’m positive Phil Plait’s answers are much better & more reliable! 😉


    The questions asked in Part 4 are:

    1) Is there another planet past Pluto or inside Mercury’s orbit?

    Me : Well yes & no.

    Not inside Mercury’s orbit no, …
    … but, yes, Eris (the erstwhile planet ‘Xena’) lies outside of Pluto’s orbit – & I’d call it and Pluto & Charon planets regardless of what the IAU in their utter lack of wisdom wrongly decided at their Prague meeting stuff-up.

    2) Why can’t we live without the Sun?

    Me : Because everything we eat (and, indeed, even our skins) need sunlight to keep us (and them) alive.

    3) How do stars form?

    Me : Well they form a number of ways. One way is to start out very rich having no talent except for looking good and promoting yourself. Then you make a … er .. how to put this ?? naughty video of yourself doing something kids aren’t allowed to do with someone you like (not necesarily love although that verb has been applied to the action …) 😉 and it “accidentally” gets out on the web & viola – a “star” like Paris Hilton or Britany Spears is born … 😉

    4) Have the constellations changed?

    Me : Yes. There used to be a constellations like Noctua & Felis (The owl & the pussycat) and like Argo Navis but the IAU in their utter lack of wisdom threw them out but kept fainter less conspicuous ones like Mensae and Leo Minor … Why? Ask them not me! 😉

    (Maybe its time for another modern revision?) 😉

    Also constellations change appearance both slowly as stars shift their relative positions over the aeons and quickly when supernovae, novae or hypergiant outbursts (eg. Eta Carinae 1840’s~ish) take place creating “new stars” where none were seen befroe.

    Plus at a very basic level if you go out at night you’ll notice the constellations change in a regular cycle from season to season and night to night as planets wander and constellations rise & set …

    5) Why is space black? Why doesn’t the Sun light it up?

    Me : The Sun does light up what’s there – & what’s there is ..well .. just space. Dark, empty, space. Light shines on things and space is No – thing. 😉

    6) Do galaxies move around in space and do they collide?

    Me : Absolutely yes! We’ve got lots of glorious images of colliding galaxies, see them in all stages of collision and merger and one day, many billions of years in the future, our galaxy too will collide with the Andromeda Galaxy (M31) and fuse to form the “Milkomeda” galaxy!

    7) What is the biggest galaxy?

    The one with the most Dark Matter and that’s what is the matter since we can’t see Dark Matter to unravel this matter of which has most matter – but fortunately it really doesn’t matter! 😉

    (We do know its not our Galaxy or any nearby one’s though …)

    Hmm .. Only 7 questions and not 10 again, BA?


    Wonder how close my answers are to his ones – I’m suspecting not very at all! 😉

    .. & Not that there’s any doubt about this but his are theopne’s you should go for folks! I know I’m not giving upmy day job anytime soon!

  16. Joker

    Uh Phil ?

    I’m really confused by your no swearing policy here & not sure what words Ican and can’t say. :-(

    Please could you post a list here of all the words I can’t say so I know not to use them? 😉 ThX!

  17. Koro

    I have to say that I don’t understand the whole “space is not infinite” thing. What happens if one were able to travel to the “edge” and then go one more foot? Or is space not infinite in that you approach a point where there are no more galaxies (i.e. a finite volume of galaxies in an infinite amount of empty space)?


  18. Kingthorin

    Answer 3, when discussing size 1/10th the size of the Sun is VERY different than 1/5th the size……”or something like that” 😉

  19. Kingthorin

    Answer #5, if the universe if finite in space then we should be able to figure out it size and where the “center” is. Which flies in the face of sooooo many other discussions that have been had around here.

  20. musiconthemoon

    Didn’t the Hubble Space Telescope point at a dark region of space to get the images of the galaxies. If this is the case doesn’t that mean that light is getting to earth but it is just really really faint and not visible to the naked eye? Thus disproving the argument that if the universe was infinite the sky at night would be bright?

    Apologies if this is a stupid question, I’m by no means an educated astronomer, just interested.


  21. I saw a request for a transcript in the comments to Part 5, so I posted a transcript over there and went ahead and wrote one for Part 4. Find it below. It’s been typed up from the video and includes ums, uhs, and stutters (of which BA has remarkably few!). Good stuff!


    Phil Plait – Part 4

    “Do you think there are any more planets in the solar system? And is there another planet in front of Mercury called Vulcan?”

    Well, there might be more planets in the solar system. It’s unlikely, but it’s possible that there are planets even as big as the Earth, or bigger, way far out–way past Pluto’s orbit! Out there they’d be really cold and really dark and they’d be moving so slowly as they orbit the sun that they’d be really hard to detect. But that doesn’t mean they’re not there. There might be a planet out there and, uh, its gravity would be so weak (because its so far away) that it wouldn’t affect the planets, uh, interior to it–the planets like Jupiter and Earth and Uranus and Neptune.

    So, there could be one out there, it’s just probably pretty unlikely. Now, as far as a planet closer to the Sun than Mercury: long time ago it was thought that maybe there was a planet inside Mercury’s orbit called Vulcan, they actually gave it a name, and because, uh, this was because Mercury wasn’t orbiting the Sun the way it was expected to. But it turns out that we didn’t understand everything about gravity back then.

    Einstein came along and redefined what gravity was and when you use his equations, his models of how gravity works, Mercury was doing exactly what it was supposed to be doing and they realized there was no planet inside Mercury’s orbit. There might be some small asteroids in there, small chunks of rock a few hundred yards across, but they’re so close to the Sun they are really difficult to see. But there are people looking for them. We don’t know if they’re there or not, but if they are people will find them and that would be pretty interesting.

    “Why can’t we live without the Sun?”

    Well, we can’t because the Sun is the source of heat and light in the solar system, and so it keeps our planet warm. It keeps our atmosphere from freezing and our oceans from freezing. Plants get light from the Sun and they’re a part of the food chain. If you were to turn the Sun off, we’d freeze and the plants would all die and everything would die, so that’s why we need the Sun.

    “How do stars form? What is the biggest and smallest star?”

    Well, the stars form from giant gas clouds called nebulae and these things collapse under their own gravity. Sometimes two of them collide–these gas clouds collide–and it makes ’em collapse and then there’re pockets where the gas gets really dense, and then its own gravity starts to compress it, and in the center it gets really hot, and then that is able to squeeze hydrogen atoms together to form helium in a process called fusion which generates energy–and that makes a star.

    And we’ve seen this! We see this process happening in these gas clouds–pictures from Hubble; pictures from ground-based telescopes–and so we know that’s how this works, and it makes stars of all different sizes. The smallest stars are called brown dwarfs (its kind of a silly name; they’re not really brown), but they have a mass of about a twelfth of the sun which is like 80 times the mass of Jupiter. And that’s sort of the minimum mass you need to be able to get the pressure and temperature y’know high enough in the center to be able to make energy this way. Um, as far as size goes they’re about a tenth of the size of the Sun, something like that, a fifth a tenth, something like that. They’re smaller than the Sun. The biggest stars have about a hundred times the mass of the Sun, and they can be huge. They can have, y’know, five or ten or fifteen times the size of the Sun physically, but when these stars use up their fuel they can expand and they can get really huge, and-and they can be a hundred million miles across, or even more! The Sun’s less than a million miles across, so you’re talking about something that’s a hundred or two hundred times the size of the Sun, These are immense objects and we see these as well, so there’s a huge size-range of stars.

    “Have the constellations changed in the last few million years?”

    Yes, they have. When you go out and look at the stars night after night it doesn’t really look like they change much, but in fact they’re moving, just very slowly. And over thousands of years, and tens of hundreds of thousands of years–millions of years!–the constellations have changed as the stars change as well. So, if you could get into a time machine and go, y’know, a million years in the future you might notice that Orion and the Big Dipper and those constellations they’ll have actually changed their shape.

    “If the Sun is so big, how come it doesn’t light the darkness? Why does space look black?”

    Well, this is one of the most basic questions you could ask. Why is the sky dark at night? a long time ago it was thought that the universe was infinite–it went on forever–and that it was infinite in time as well–it existed forever. And if that were true, you’d expect space to be really bright because no matter where you looked in any direction you’d eventually see the surface of a star and that would all add up. It would be like sitting inside of a star, really, and so space itself would be as bright as the Sun.

    But it’s not. It’s black. And that’s because the universe is not infinite either in space or time. It has an age. It started in the Big Bang about 13.7 billion years ago, so it’s not infinite in time, and it’s finite in space as well–it doesn’t go on forever. So, you’re never looking far enough to always see the surface of a star, and the universe hasn’t existed long enough for it to be infinite in space. And so that’s why it’s not as bright as a star, and it’s also dark because it’s not filled with stuff.

    During the day the sky is bright because there’s air, uh, on the Earth and that takes the sunlight and spreads it out and so you see light coming from every direction. But space is empty. Its like a vacuum (not really, but close enough), and so when we look out in space we just-we just don’t see anything. And so that’s why its black. There’s just not a whole lot there.

    “Do galaxies move around space? Do they ever collide? Will our galaxy collide with another one? When?”

    Well, galaxies do move. Galaxies are collections of billions of stars and they have gas and dust in ’em and they take all kinds of cool shapes. They can be like football shaped, or like balls, or they can be flat and have spiral arms, and they move around sort of like the way planets move around the Sun. Galaxies can orbit each other. As a matter of fact, we see clusters of galaxies that can have hundreds of even thousands of galaxies in ’em, and their own gravity holds ’em together, and its kinda like a beehive with all the bees going all around.

    The galaxies all orbit each other, but that means that sometimes they collide. Galaxies actually smack into each other, and they can pass through each other actually. Stars are so small and far apart that galaxies can actually pass right through each other without actually having any stars collide. But their gravity kinda holds them together and the galaxies’ll merge and they might form sort of a ball shaped thing or-or whatever, but the Milky Way is actually going collide with the Andromeda galaxy (which is a spiral like ours), but that’s not going to happen for a billion years, so don’t wait up nights for it. It’s going to take a long time, but eventually we will collide and form a-a more massive, probably elliptical galaxy. (transcriber’s note: BA seems disturbingly enthusiastic about this prospect.)

    “What is the biggest elliptical galaxy?”

    Y’know, I don’t know, and I looked this up, and I was having a hard time finding what the most massive–what the biggest elliptical galaxy was. There is a galaxy called M-87. It’s-it’s relatively close by. It’s about 60 million light years away. That’s a long way! But as galaxies go that’s-that’s pretty close, and it has about a trillion times the mass of the Sun. Our Milky Way galaxy has about two hundred billion times the mass of the Sun. So, M-87 is five times more massive than our galaxy, and our galaxy’s considered to be pretty big. Now, there are probably galaxies out there bigger than M-87, but not many. It’s really one of the most massive galaxies out there. So I’m thinking roughly a trillion stars is gonna be the number of the stars in the most massive galaxy.


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