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Stars and/or Comets

Date: Tue Apr 23 08:58:50 1996
Posted by:
Grade level: 4-6
School/Organization: Rye Neck Middle School
City: Mamaroneck State/Province: New York
Country: USA
Area of science: Astronomy


How big can a comet be in size? How fast can a comet go? Have you found a new comet? Have you found a new star? How close can you get to a star with out getting hurt? How hot is the hottest star? Approx. how many stars are they in the sky? How fast is the fastest comet? Has there been any big discoveries lately,please keep us informed. Respond as soon as possible(A.S.A.P.)

That's a lot of questions! I'll answer them in turn:

1) How big can a comet be in size?

Questions like "How big can something get..." usually run into trouble simply because of definitions. The latest thoughts about comets are that they are chunks of ice and rock something like a snowball with gravel in them. The most famous comet of all, Halley's , is about 16 x 8 x 8 kilometers in size. Hyakutake, the bright comet that swung by the Earth in March 1996 is probably somewhat bigger then that. Hale-Bopp, which promises to be extremely bright by the end of 1996 may be even bigger still.

But how big can they get? Just last year, the Hubble Space Telescope may have captured pictures of truly giant iceballs, that may be as big as 1000 kilometers across or more! Some people even think the planet Pluto is more like a comet than a planet. So comets may get pretty big, but our definition of just what is a comet may break down before then!

2) How fast can a comet go?

Some comets fall towards the Sun from very far away, even way beyond the orbit of Pluto. It is possible for those to pass the Sun at speeds of about 600 kilometers per second! For comparison, the speed of sound on the Earth is about 1/3 kilometers per second.

3) Have you found a new comet?

I personally have not. Most comets are not discovered by professional astronomers at all, but are found by amateurs! This is because most professional astronomers look at very specific parts of the sky to see a specific object, while amateurs scan the heavens looking at lots of different things. Also, amateur 'scopes usually can see much larger parts of the sky, and so they cover more "ground". There is a professional- based comet and asteroid hunting program, called Spacewatch. They look for objects that get very near to the Earth.

4) Have you found a new star?

Actually, I have. Using Hubble Space Telescope images of a star that blew up (called a "supernova") in 1987, Lifan Wang-- an astronomer now at Texas-- and I noticed a star near the supernova that had never been seen before. It appeared to be sitting right on top of a ring of gas that circles the star that blew up, but we think that is coincidence. It really is not in the ring, but it just happens to line up that way. The star is about one million times fainter than the faintest star you can see with your unaided eye!

5) How close can you get to a star with out getting hurt?

Well, suppose you are in a space suit that lets you breathe, but does not protect you from the heat. If you sat in full sunlight near the Earth, you would freeze! The temperature is a little bit below the freezing point of water this far from the Sun (the Earth's atmosphere keeps us warm). A rough calculation shows that by the time you get to the orbit of Venus, your temperature will be a bit higher-- about 20 degrees Centigrade, or maybe room temperature. At Mercury's orbit, the temperature would be well above the boiling point of water-- as a matter of fact, the temperature near Mercury is enough to melt lead! So for the Sun, you could get somewhere between Venus and Mercury before getting in trouble. That translates to maybe 90 million kilometers. Of course, lots of stars are hotter than the Sun...

6) How hot is the hottest star?

One of the hottest stars on record is a small dense star called a white dwarf. It was once much like the Sun, but when it grew old it got very big and cool and turned red (this is called the "red giant phase"). After that it collapsed, getting very hot. This particular star, in the center of a type of object called a "planetary nebula" may be as hot as 200,000 Centigrade! The surface of the Sun is only about 6000 degrees, for comparison. For a short time, some stars get hotter than that. The center of a supernova, or exploding star, can be over a billion degrees! But only for a VERY short time.

7) Approx. how many stars are they in the sky?

You can see about 6000 stars with your unaided eye if you live on the Equator. From the United States, you can see maybe 4000 or so over the course of the year (the Sun is always blocking some and you have to wait for the Earth to go a little bit around the Sun to unblock them). With a telescope you can see LOTS more. The Hubble Space Telescope has a catalog of over 15 million stars, and those are just the bright ones! In our Galaxy, the Milky Way, there are something like 200 - 400 billion stars. And there are billions of galaxies like ours in the Universe. It's a big place.

8) How fast is the fastest comet?

This is the same as question (2), I think.

9) Are there any new discoveries lately?

Always! A new planet was found recently orbiting another nearby star. A team of astronomers, which had previously found two other planets, has found another which may be about the size of Jupiter.

The Hubble Space Telescope publishes new images every few days or so, and they have recently compiled some nifty stuff Hubble has done in recent time. Check out the WWW link above to see them.

©2008 Phil Plait. All Rights Reserved.

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