Astronomy Q&A game

[cragwolf]

here is my guess: tholin

(my post has to be longer than ten words for some reason.)

 

[SchrodingersCat]

Titan tholins?

 

[schwarzchildradius]

Manna from Heaven?

 

[Phobos]

cragwolf got it first...your turn

(welcome to Physics Forums, SchrodingersCat!)

 

[cragwolf]

Cepheids

Here is my question (I hope it hasn't been asked before):

Cepheid variable stars pulsate in a very regular and stable manner. What causes this pulsation? Please include in your answer an explanation of why the Sun doesn't pulsate in this way.

 

[schwarzchildradius]

Here is my question (I hope it hasn't been asked before):

Cepheid variable stars pulsate in a very regular and stable manner. What causes this pulsation? Please include in your answer an explanation of why the Sun doesn't pulsate in this way.

Typically there's some Helium in the atmosphere of the star. As radiation ionizes the Helium, the atmosphere expands and the star brightens. The Helium cools and loses its ionization, falling back toward the center of the star, causing it to dim a bit. The correlation between the period of the brightening and the intrinsic brightness of the star allows you to estimate the absolute distance.

 

[cragwolf]

Typically there's some Helium in the atmosphere of the star. As radiation ionizes the Helium, the atmosphere expands and the star brightens. The Helium cools and loses its ionization, falling back toward the center of the star, causing it to dim a bit.

Assuming that's true ( ) , why does the atmosphere expand when the Helium ionizes (and contract when it recombines). Also why doesn't this happen to the Sun?

 

[schwarzchildradius]

When Helium is ionized it becomes opaque to radiation, the radiation causes the envelope of Helium to expand as it is heated. When the Helium de-ionizes it becomes transparent to radiation & contracts. The sunis not massive enough to be a Cepheid variable

 

[cragwolf]

Well done. Your turn to ask a question.

 

[schwarzchildradius]

Super! Here it is: What's the name of the Egyptian mathematician who first calculated that the world must be round, and what visible clue did he use to find this out? (If you've seen "Cosmos" by Sagan this one is easy)

 

[SchrodingersCat]

Eratosthenes observing the noontime Sun.

 

[SchrodingersCat]

Sorry, that was for measuring the circumferance. I believe it was Aristotle using Earth's shadow on the Moon during a lunar eclipse

 

[schwarzchildradius]

Naw you were right with the first one. your go/

 

[schwarzchildradius]

By the way, I've run across a passage from Cosmos where he names the hydrocarbon sludge
"Such experiments were first performed in the early 1950's by Stanley Miller, then a graduate student of the chemist Harold Urey. Urey had argued compellingly that the early atmosphere of the Earth was hydrogen-rich, as is most of the Cosmos; that the hydrogen has since trickled away to space from Earth, but not from massive Jupiter... After Urey suggested that such gasses be sparked, someone asked him wat he expected to make in such and experiment. Urey replied, "Beilstein." Beilstein is the massive German compendium in 28 volumes, listing all the organic molecules known to chemists."
So what is "Tholin?"

 

[Phobos]

So what is "Tholin?"

The hydrocarbon sludge on Saturn's moon Titan (discussed briefly in Sagan's sequel to "Cosmos" called "Pale Blue Dot")

 

[SchrodingersCat]

Sorry It took so long to post this. Anyhow, here is the question. What supports a Neutron Star from collapse? (Do not just name the type of support but also explain it)

 

[Labguy]

Sorry It took so long to post this. Anyhow, here is the question. What supports a Neutron Star from collapse? (Do not just name the type of support but also explain it)

It is called "neutron degeneracy pressure". Neutron degeneracy is actually at a "core", since a neutron star is not all neutrons from the surface down, they (the stars) actually have a structure. the "mass" is the gravitational mass and is distinct from the baryonic mass, because the gravitational redshift of a neutron star is so great, the gravitational mass is about 20% lower than the baryonic mass.

When the density towards the inner core exceeds the nuclear density of 2.8x10^14 g/cm^3 by a factor of 2 or 3, exotic stuff might be able to form, like pion condensates, lambda hyperons, delta isobars, and quark-gluon plasmas. To me, most likely quark-gluon plasmas. So it is essentially the strong nuclear force in the deep interior preventing further collapse.

The interesting structure can be seen at: http://www.lsw.uni-heidelberg.de/users/mcamenzi/NS_Mass.html

Beyond that (simple explanation) it gets a bit complicated, and several models all work to show one form or another of "condensate" to prevent collapse without additional mass being added.

 

[SchrodingersCat]

Well Labguy, it is your turn to ask a question.

 

[Labguy]

Well Labguy, it is your turn to ask a question.

Hey, S-Cat. I'm going to have to pass and let you ask another because I'll be on a trip and "away from a computer" for several weeks.

Ask another Q or pass to the last guy, etc. I'll be back for more in a few weeks.

 

[SchrodingersCat]

In a cluster most stars have the same main sequince turn off point. However, there is another class of bluer stars that seem to be younger. The formation of these stars is uncertain but one of the theories is that they form when one star accretes matter from another star. Another possible formation scenario is the collision of one or more stars. If you look at the HR diagram for some clusters you will see the that most stars lay along the same main sequice and horizontal branch and whatnot but there is also often a separate main sequince and horizontal branch (containing much less stars) for these strange objects.

What is the name of these objects, who was the first to discovere them and which globular cluster were they first found in?

 

[SchrodingersCat]

Need a hint?

The person who discovered these objects was one of the co-discoverers of the quasar.

I guess if no one gets this with in the next couple of days I should post a new question? Or should I just try and think of more hints?

 

[Cecil]

I believe these stars are termed "blue stragglers". They are stars in a globular cluster that are on the main sequence, but to the left of the turn-off point. This means that they should have already burned out into red giants, but haven't yet evolved. In some sense, they appear to be normal stars, but younger than the other stars in the cluster. They were first found in M3 by Sandage in 1953 (had to look his name and date up, couldn't remember them OTOH).

A coalescion of two smaller stars that have not yet evolved would produce a such a blue straggler because, of course, more massive stars tend to be bluer and brighter. Mass transfer from a nearby binary "overflowing" its Roche lobe works in the same way to shift a star left-wards on the main sequence.

Of course, these stars exist everywhere in the universe, but they are only easily identified as BSs in a globular cluster due to the proximity of stars of precisely the same age.

Good question, and mine will follow as soon as I can think of one.

 

[Cecil]

Okay. Is it possible for a pulsar to spin so fast that it breaks up, and are there any known mechanisms for this? How fast (% of c) does the surface need to be going for this to happen?

 

[FZ+]

If it spins so fast... why would it form in the first place?

 

[SchrodingersCat]

Congrats on your answer to the last question Cecil.

Well mv^2/r=GMm/r^2 (centripetal force= gravity) sets this limit. Thus v=Sqrt[GM/r] where G is the gravitational constant. I will assum M~1.4Msolar and r=10km.

=> ~0.46c.

I believe another factor which actually occurs at lesser velocities involving gravitational radiation. This effect seems to not allow the pulsar to spin so fast and thus saves it from flying apart in such a manor.

Nice question!


FZ++ Well, it spins fast to conserve angular momentum. Neutron Stars have radii of 10e5 cm. Our sun for example has a radius on the order of 1e10 cm. This dramatic loss in radius causes it to have to spin faster in order to achieve conservation of angular momentum.

 

[SchrodingersCat]

Sorry I have not been around for a while. I have been really busy. Umm... Cecil, do you want to ask another question? I am afraid I prolly will not be posting for a bit as I have a lot of other stuff to attend to.

 

[Shadow]

is that a no cecil?