Astronomy C

[walkingstyx]

Also quite a few HR Diagrams have information of this nature already written on them. My personal favorite:
http://www.slackerastronomy.org/slacker ... iagram.png
It comes complete with mass, lifetime, and radius in addition to the normal. And it's pretty.

[sisomg11]

Thanks to those answering those questions: now for another. At our last invitational, we got our test back but we haven't seen an answer key. A few of the questions that we missed I'm not really sure how to do (because we thought we did them right and got major points off on them - and still by some miracle managed a silver), so could someone explain these three to me?

1) Careful observations over the course of many years have shown that there seem to be stars orbiting Sagittarius A*. The period of the orbit of one of these stars is measured as 16 years. There is no chnage in the radial velocity of the star over the period of its orbit, and the motion of the star describes a circle with an angular diameter of .24" (arcseconds). For the purposes of this problem, we assume that the distance to Sagittarius A* is 8.3Kpc. What does this tell us about the mass of Sagittarius A*?

2) Careful observations of a number of RR Lyrae stars in this object (attached to a picture of Andromeda) yield an average visual apparent magnitude of mv = 25.3. What is the distance to this object?

3) (attached to a time plot of apparent magnitude in v, b, and r bands) - How far away is this star and by association the DSO that contains it? (The period is given as 13.87 days and it is a Type II Cepheid)

[Glacierguy1]

for number 2, the magnitude of all RR lyrae stars is about 0.7, sojust use that number in the distance modulus.

for number 3, use the v band apparent magnitude and you can find the absolute magnitude using the period luminosity relationship. then use the distance modulus.

I think for number 1, you can use either the small angle approximation or a tangent formula to find the semimajor axis of the star's orbit, then convert it to AUs and use newton's form of kepler's 3rd law. then it will give you an answer about 3.6E6 solar masses and you can neglect the mass of the star next to the black hole.

[sisomg11]

Can you give me a more detailed explanation on number 1? I understand now on the others, but that one was a bit confusing still.

[Glacierguy1]

Draw a triangle with the earth at one vertex and the two positions of thes star at the other points. the angle at point earth is .12 seconds and the side conecting earth to the original positon of the star is 8.3 kiloparsecs. use the formula tangent = opp./adj. and the tangent of .12 arcsec times the adjacent side 8.3 kiloparsecs. once you get the answer, about 1000 AU, you can plug it into Newtons version of kepler's third law. It should give you about 3.8-3.9 Million solar masses, comparable to accepted values for the mass of Sgr A*

[andrewbji]

Out of curiosity, do you think there will be a lot on the physics portion on black holes? I'm thinking that they might ask something about the Schwarzchild Radius, but I can't think of other stuff. Is it possible to find the mass of a black hole (I thought it was infinite since it has the singularity, so just infinity)?
Also, I read from a book that black holes can change into a neutron star, is that true? I was kind of confused, because I thought once a black hole, always a black hole.

Finally, I got this question, does anyone have any ideas on how you figure it out: In the absence of extinction an O5 star has colors U-B=-1.1 and B-V=-0.3. Its absolute magnitude is MV=-5.6. Interstellar extinction has extinction ration A(B)/A(V)=1.34 and A(U)/A(V)=1.57. An O5 star is about 105 times more luminous than the Sun and has a radius that is 10 times larger than that of the Sun. What U-B and B-V colors would an O5 star be if detected behind a molecular cloud with extinction in V band of A(V)=3.0

[naureeng309]

Hello everyone !

I'm planning on doing the Astronomy event. I took it this year at Regionals and I did not do well =( There was a lot of conceptual stuff I didn't know. The calculations weren't too bad.

I want to do a lot better next year. What books and sites did you guys use to study astronomy? Any good links =]? Also, how did you guys study the deep sky objects? I have trouble identifying them based on images (I'm a total newbie lol). So any tips on how to get started ?

[walkingstyx]

Finally, I got this question, does anyone have any ideas on how you figure it out: In the absence of extinction an O5 star has colors U-B=-1.1 and B-V=-0.3. Its absolute magnitude is MV=-5.6. Interstellar extinction has extinction ration A(B)/A(V)=1.34 and A(U)/A(V)=1.57. An O5 star is about 105 times more luminous than the Sun and has a radius that is 10 times larger than that of the Sun. What U-B and B-V colors would an O5 star be if detected behind a molecular cloud with extinction in V band of A(V)=3.0

This question relates to color and dimming. The u-b and b-v values tell you what the color of the star is if there is nothing between it and you, but there is a molecular cloud, which obscures a lot of the light in the visible (v) range, but not much in the b or u ranges. This makes your value for v decrease by three. Then you use the ratios to figure how much you lose in the the other bands, i.e.. in the b band you lose 3*1.34 or a little less than 4.5, and in the u band you lose 3*1.57 or a little more than 4.5. You then use these to find the net change in the values, e.g. change in b-v= change in b minus change in v, and add them to the original values.

You will probably never see a question relating to color on the nationals test, so don't worry if you're a little confused. Also, I don't think you understood the idea of a singularity properly. It means that a blackhole has no volume, and thus has infinite density, despite having a finite mass. In fact you use this mass to calculate the swarzchild radius (which is the radius of the event horizon, not the blackhole itself). I would be surprised if you did not see anything relating to blackholes on the physics portion of the test. Also, I don't think blackholes can change into neutrons stars. They can decay, but I think they just slowly disappear rather than turn into neutron stars.

Naureeng390: have you checked the nationals website? It has excellent resources, including an online textbook, and lots of other helpful things. Use the powerpoint from the coaches clinic (http://www.tufts.edu/as/wright_center/p ... linic.html) as a starting point for assembling your collection of star pictures.

[saturnian]

A study guide you'll love: http://dingo.care2.com/cards/flash/5409/galaxy.swf

[daddydolphin]

its my school 4th time but riverwood allways wins eveytime we go