A box of jewels

By Phil Plait | April 18, 2006 9:58 am

No man is an island, as they say. But no galaxy is an island Universe either.

That’s what galaxies are sometimes called, when astronomers try to get poetic. A galaxy is really a collection of stars, gas, and dust, held together by their mutual gravity. Our Galaxy, called the Milky Way, is shaped like a vast, flat disk, like a dinner plate. Now take a scoop of mashed potatoes and plop it in the center. That’s the central hub, a bulge of stars. Take a pea and place it halfway from the center of the plate to the edge. That’s about where the Sun is, though on this scale the Sun would be submicroscopic: the Galaxy is 100,000 light years across, or about 1,000,000,000,000,000,000 kilometers. That’s kind of a long way.

That scale is dwarfed by intergalactic distance, too. The nearest big galaxy is about 2.9 million light years away. The most distant objects seen are over 13 billion light years away.

But we’re not alone in the dark. The Milky Way has several companion galaxies, smaller dwarf galaxies that are bound to us by our gravity. Two of them are easily visible to the unaided eye if you happen to live south of the Equator. They are called the Magellanic Clouds, because they were first noted (by Westerners I should add) by the explorer Magellan.

The Small Magellanic Cloud

To the eye they are faint, glowing patches in the sky. With binoculars you start to see that they have shape, and contain bright spots of light. To the telescope, they reveal themselves to be galaxies in their own right: massive collections of stars and gas.

They are not like the Milky Way. For one thing, they are actively and aggressively making stars; the rate of star birth in the Clouds is higher than in the Milky Way. Images of the Clouds reveal tremendous regions loaded with dense gas clouds. Stars are born in these giant gas clouds, and it’s rare that they’re born one at a time. Instead, they’re created en masse, churned out thousands at a time. Sometimes, their own gravity keeps these newly born stars together, bound into what astronomers call open clusters.

The image at the top of this page — taken by Hubble, and released to the public just this morning– is of one such cluster, called NGC 290. It’s in the Small Magellanic Cloud, so it’s about 200,000 light years away. At 65 light years across, it contains many thousands of stars. The brightest of these are already starting to die, and have turned red as they’ve expanded and cooled, preparing to explode someday as supernovae.

Astronomers like open clusters for a lot of reasons. For one, all the stars are at the same distance, so if two stars appear to be the same brightness, it’s because they really are the same brightness. This isn’t always the case; when you see a bright star in the sky, it might be an intrinsically faint star, but be very close by, while a fainter-looking star might be a powerhouse located very far away.

Another reason is that all the stars in a cluster were born at about the same time, so they are all about the same age. That means when we see difference between two stars, it’s not because one is much older than the other.

Third, the stars were all born from the same cloud, so they have about the same chemical content– they have the same amount of hydrogen, helium, iron, calcium, titanium, and so on. This is important too, as small changes in the amounts of some of these elements can really change the way a star lives its life (hmmmm… something like us, as well). Manganese, for example, is very good at absorbing light, so if a star has a little bit extra manganese its heat from the interior gets trapped, making the star hotter. It doesn’t take much extra, either, so life is a lot easier for a scientist when she studies a cluster. The stars tend to be better behaved.

NGC 265 and NGC 290 in the Small Magellanic Cloud

Then, too, scientists can compare clusters to each other, to look for similarities and differences. Looking at our companion galaxies is a good way to do this, because again the clusters are at about the same distance. They may be different ages and have different chemical compositions, but at least we can ignore distance issues. That helps a lot.

When I see a cluster like NGC 290, I know we can learn a lot about it; how stars are born, how they live, how they die. But I also look at it and think, "Wow, that is absolutely gorgeous!" The colors, the patterns, the contrast between bright stars and deep space.

Another thing, too: — the images of the clusters above are from Hubble, which observed them in the Small Magellanic Cloud in November 2004. This is the same time I was in Australia, and seeing the Clouds with my own eyes for the very first time. For my PhD I studied a star in the Large Magellanic Cloud which exploded in 1987, so seeing them for myself for the first time was very moving for me. It was an incredible experience.

There is beauty in observing the cosmos, and there is beauty in knowing it, too. And there is plenty of room for both, I think.’

CATEGORIZED UNDER: Astronomy, Cool stuff, Science

Comments (32)

  1. Professor BA wrote:

    Our Galaxy, called the Milky Way, is shaped like a vast, flat disk, like a dinner plate. Now take a scoop of mashed potatoes and plop it in the center. That’s the central hub, a bulge of stars.

    …and in the middle of the mashed potatoes, push a black olive. That’s a black hole of more than 2 million solar masses!

  2. and some thick gravy for the dark matter…

  3. I am surprised that you didn’t talk about the large magellanic cloud. the interesting thing about it is that the large magellanic cloud was a captured galaxy which is currently being stripped of it stars. The more interesting part is that the large magellanic cloud used to be a spiral or a barred spiral galaxy. you can see the distorted spiral on most pictures. It is unknown of it used to be a spiral or a barred spiral because the distortion is too great.

  4. Actually, the supermassive black hole in the center of our Galaxy is more like 4 million solar masses…

  5. wow… I remember the 1987 supernova, I was living in Chile that year, so had front row tickets for the show!

  6. Thanks, Professor. I suppose the mass got revised upwards between the link you gave and the link I found (curse this silly Internet for not updating itself). “More than two million” is technically consistent with “more than four million”, of course, except for very large values of two.

    At any rate, that’s a lot of mass in a small space. If I can trust a back-of-the-envelope calculation I just made, the Schwarzschild radius of that BH must be about 12 gigameters, or 0.08 AU. Four million solar masses in a sphere less than a tenth the radius of the Earth’s orbit — yikes!

  7. Speaking of Dark Matter, the Australian magazine-style science program, Catalyst has a report on Dark Energy. It was discovered that the light from distance super novae is being accelerated on it’s way to the Earth. And I thought the speed of light was constant.

    Phil, can you shed any light on this discovery?

  8. Nobby

    Someone once told me that they didn’t believe that there was any room in science for faith, he didn’t believe that miricles existed. The birth of stars, the fact that we can appreciate them and marvel at the universe we live in, okay, we can explain them, but does that make them any the less miraculous? Or neat?
    With the Schwarzchild radius being that big, how bad would the tidal forces be?

  9. Dan, try here. This discovery is a few years old, actually.

  10. Beautiful images, beautiful discriptions. But I do have one nagging question: in the large version of this cluster there are several stars with “lens-flares”. That is, they have rings around them. My question is: what causes some stars to have this flare and others to not ahve the flare.

    Based on the image I would guess a few things:
    - It doesn’t appear to be based on the brightness of the star
    - It DOES appear to be based on the color of the star

    So, astronomers reading this post, is there a reason we sometimes see flares and sometimes don’t?

    Thank you!

  11. Jamie

    I may be way off base here, but what I know about lens flare comes from Earth-bound photography. Lens flare generally occurs when you have an extremely bright light source in the frame of the picture, usually if it’s head-on flare is less of a problem than when the light source is shifted to the periphery of the frame, allowing for distortion as the light passes through the various lens elements. Again, this might not translate into astronomical photography, but it might help!

  12. If the galaxy is the size of a dinner plate, then the next nearest galaxy would be about thirty feet away. So, probably in your bathroom, if you set the Milky Way on your dinner table.

    Huh, that’s closer than I would have thought. I wonder how far it would be to the next galactic cluster…

  13. KingNor

    most delicious analogy every.

  14. Interesting, and eloquent, as always!

  15. Kaptain K

    “the interesting thing about it is that the large magellanic cloud was a captured galaxy which is currently being stripped of it stars.”

    This is also true of the SMC and several (all?) other dwarf galaxies that are companions to the Milky Way. In fact, it is thought (by some) that some of the larger globular clusters, such as Omega Centauri, may be the cores of dwarf ellipticals that have been stripped of all their outer stars. Eventually, all of the dwarf companions will be absorbed by the MW.

  16. Hmmm…I measured from my dinner table to my bathroom. It was 32.2 feet. So the next galactic cluster must be the feet between my bath room and …hmmm…my potting shed in the back yard? The garden? The front driveway? I am all confused now…Enlighten me. You guys are all so smart. Alas…I am only a lowly psycho-Therapist, Minister, Gemologist and NASCAR wannabe journalist. If you could explain it better to me in terms of psychosis, meditation, cwt, or camber I would understand it better.
    I do love your blog, dear Prof.

  17. RAD

    Numbers don’t lie although they sure can be hugh! i need a better telescope and more importantly more practice finding stuff. I can find stars and planets easy enough but that dang moon is hard to get in my telescope. Give me a motorized one!!!

  18. Amy

    Just a thought regarding the balloon analogy: the opening where you blow the air in means the balloon is not entirely closed.

  19. Berkeley

    Anyone know when the next supernova show is on? And where to get tickets?

  20. Mike Barron– they aren’t really lens flares. Those rings and spikes are called diffraction patterns, and they do in fact depend on the color of the star. Different colors of light behave a little bit differently, especially when they interact with a telescope. Red stars have larger diffraction rings, so you see a red ring around a blue one in the Hubble images. Sometimes the red light gets smeared out more inside the detector itself, changing its shape, too. It gets pretty complicated. Try a google search to find out more.

  21. Irishman

    Amy said:
    >Just a thought regarding the balloon analogy: the opening where you blow the air in means the balloon is not entirely closed.

    That may either be a really important observation, or it may be stretching the analogy beyond it’s limits. I think the latter.

    Nobby said:
    >Someone once told me that they didn’t believe that there was any room in science for faith, he didn’t believe that miricles existed. The birth of stars, the fact that we can appreciate them and marvel at the universe we live in, okay, we can explain them, but does that make them any the less miraculous? Or neat?

    Define “miraculous”. I would agree to “amazing”, “wonderful”, “awe-inspiring”, “beautiful”, “thought-provoking”, and probably a dozen other descriptive words, but not “miraculous”.

  22. Jim Rocko

    Only once have I seen mentioned the fact that the Radiation belt would pop any human like popcorn, not equipment just living beings. With that in mind, and the admission from the government that we still need to build a craft that can safely get man to outerspace, should be a clue that we haven’t yet gotten there. Untill we find a way to protect living beings from severe (microwave like) radiation in the belt we are destined to never have man go anywhere except sub orbital. And just one picture comparison of two different so called moon landing sites that match up exactly should be enough to convice even the most die hard believers that it was not real.

  23. And the award for shockingly original and relevant comments goes to. . . .

  24. I’m sorry; that was harsh of me. (I’d say it again if I had the chance, but I’ve never considered myself a kind person. Nyah.) So, before we go any further, I assume everyone here has read the following pages?

    On “the deadly radiation of space”: http://www.badastronomy.com/bad/tv/foxapollo.html#radiation

    And on “identical backgrounds”:
    http://www.badastronomy.com/bad/tv/foxapollo.html#backgrounds

    Good? Now we can talk.

  25. RAD

    Jim Rocko(does balboa fit in here) even in you comment I think you miss an important set of words in building a craft that can get us safely to outerspace. That doesn’t imply we didn’t get there just we need better safety. Isn’t that always a goal? It is at my work, we even have captain safety. I would actually say that there is a pretty good safety record at the space program, am I wrong here? I just want to know if I can take along a bag of microwave popcorn to cook for the ride

  26. Nigel Depledge

    To elaborate on RAD’s comment : with the Apollo missions, I don’t think there was one that didn’t have problems of some kind or other. It was only Apollo 13 that had a really serious, life-threatening problem.

    Anyway, Jim, why should something you read somewhere once carry more weight than the pictures and moon rock returned from the moon by the Apollo astronauts?

  27. Nigel Depledge

    Back on topic:

    Phil, another great pic brought to our attention and another great post to go with it. Thanks.

    I love it when you wax lyrical about the stars in our sky.

  28. Irishman

    Jim Rocko, you are completely misinformed.

    Read this please:
    http://www.clavius.org/envrad.html
    >”The recent Fox TV show, which I saw, is an ingenious and entertaining assemblage of nonsense. The claim that radiation exposure during the Apollo missions would have been fatal to the astronauts is only one example of such nonsense.” — Dr. James Van Allen

    That’s right, the Dr. Van Allen who discovered the belts and named them.

    Needless to say this is a very simplistic statement. Yes, there is deadly radiation in the Van Allen belts, but the nature of that radiation was known to the Apollo engineers and they were able to make suitable preparations. The principle danger of the Van Allen belts is high-energy protons, which are not that difficult to shield against. And the Apollo navigators plotted a course through the thinnest parts of the belts and arranged for the spacecraft to pass through them quickly, limiting the exposure.

    As for “the admission from the government that we still need to build a craft that can safely get man to outerspace”, you haven’t given the context for that statement. Was that really in regard to radiation, or the concerns over the Shuttle External Tank foam shedding and the delicate orbiter tiles and possible damage? That’s a very vague statement that could mean a lot of things, certainly not requiring the conclusion that we haven’t sent people into space.

    Even if the statement was in regard to radiation, there is a lot of different radiation concerns to consider. One is the Van Allen belts, but the links provided address that concern. Another is Solar Particle Events – i.e. solar flares. Those provide bursts of strong radiation that if aimed in the correct position could kill humans in space. And no, the Apollo missions weren’t really protected against them. Shocker, but true. Rather, solar particle events are detectable by means of variations in the Sun that occur prior to the event.

    http://www.clavius.org/envsun.html

    The protection was adequate for the Van Allen belts and normal particle flux from the sun, but probably not enough to protect against a major solar event. It would have indeed been prohibitive to supply the Apollo spacecraft with the shielding necessary to ward off solar event radiation entirely. But with the shielding provided, the astronauts would have been able to withstand a major solar particle event for as long as two hours without receiving a lethal dose.

    But protection against radiation isn’t always a matter of piling up enough material to weather the storm. Sometimes it’s a matter of planning and evasion.

    A major solar event doesn’t just cut loose without warning. It is possible to observe the “weather” on the sun and predict when a major event will occur. And this is what was done on the Apollo missions. To be sure, the missions were planned months in advance and the forecasting was not that farsighted. But they would have had enough warning to call off the mission should a solar event have started boiling up from the depths of the sun.

    Statistical probability was the main protection for the Apollo crews. The forecasters would have been able to rule out major events during the first few days of the mission. And so out of a nine-day mission that might only leave five or six days of vulnerability. The chances of a major solar event occurring within a given five-day period is quite remote, even during periods of exceptional activity.

    Also note the difference between a few day long trip to the Moon and a two or more year long trip to Mars. The risks and exposure probabilities are not equivalent.

  29. Irishman

    Hmm, that post might be a bit misleading. Only the top quote is from Dr. Van Allen. The rest of the quoted material is from Clavius, the website run by Jay Windley.

  30. Robert Madewell

    Thanks for the article about star clusters. I have been enjoying looking at m44 through binoculars the last few weeks. I live in a rural area so I really look forward to clear nights.

  31. schwa sticker

    Nobby:
    The birth of stars, the fact that we can appreciate them and marvel at the universe we live in, okay, we can explain them, but does that make them any the less miraculous? Or neat?

    Miraculous: yes. Neat: aw hell no!
    Anything which is permitted by the laws of the universe is, by definition, NOT miraculous.

    With the Schwarzchild radius being that big, how bad would the tidal forces be?

    from: http://casa.colorado.edu/~ajsh/schw.shtml
    “The tidal force goes as M / r3 at distance r from a black hole of mass M.”
    and
    “rs = 2 G M / c2″

    Since the Schwarzschild radius increases proprotionally to the mass if you double the mass, you double the radius.

    If you double your distance from the singularity, your tidal force drops off by a factor of 8 (2^3)

    Therefore, for each mass doubling, the tidal force at the event horizon would be quartered. (Tidal force@event horizon: inversely proportional to the square of the mass)

    If my mass is correct, I think the tidal force at the Schwarzschild radius of the sun would be 4 million squared times as powerful as at the event horizon of the galactic core? 16 trillion times weaker than at the supermassive black hole…it sure is a good thing for the people living on the surface of the sun that it doesn’t come anywhere near fitting in its Schwarzschild radius!!

  32. “the Galaxy is 100,000 light years across, or about 1,000,000,000,000,000,000 kilometers. That’s kind of a long way.”

    Yes, but practically speaking, how would that compare, time-wise, to driving from LAX to Santa Monica during rush hour?

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