Astronomy C

[mathemaniac]

Does anybody have a good reference for galaxy morphological classification? Or classifications of AGNs, galaxy clusters, quasars, etc. if they exist? That's the only thing that's bugging me right now, because there are a bunch of good sources to find out information on the DSOs and mathematical formulas that we might need but I can barely find anything about classifcation.

[smartkid222]

http://en.wikipedia.org/wiki/Galaxy_mor ... sification
most important are the types of galaxies on hubble's tuning fork for normal galaxies.
For active galaxies the classification is more difficult...

[crazy77]

last year it was galaxies and the two+ years before that it was variable stars.

So knowing that (this could be a really stupid question), will the event be drastically different than a year or two years ago cuz I'm looking at some tests right now from the past two years and wondering if they are good representations of what the test will be like

[smartkid222]

from last year? probably, except for differenct DSOs of course
from 2+ years ago? perhaps not.

[crazy77]

What does a cepheid's spectrum look like? I'm looking at a test where they asked that and I can't seem to find a picture online of one

Edit: Also how do you find the distance to a cepheid from Earth given the log of its period and its absolute magnitude? And up to what distance can cepheids be used to determine its distance from Earth? One source I read said 13 million while another said 130 million (maybe one was a typo?) Sorry for all the questions but much thanks to whoever answers them!

[TheBalticSea]

@ crazy77: To find a star's distance, you can use the apparent luminosity and absolute luminosity with the distance modulus formula (http://en.wikipedia.org/wiki/Distance_modulus).
Since you only have the absolute luminosity and the log of the star's period in the scenario you mentioned, you need to use a Cepheid Period-Luminosity chart to infer the apparent magnitude (http://outreach.atnf.csiro.au/education ... heids.html). <-- that one was calibrated using stars in our galaxy
use that chart to find apparent luminosity and plug into distance modulus.

[crazy77]

Thank you so much! That link is really helpful.

I actually read the question I was talking about wrong though so basically they actually gave you the apparent magnitude but the only other information they give you is this exact picture: . They give you the log of the period and the absolute magnitude of OTHER "objects of this type", but not the actual object itself. Would you have to guestimate the period in this example and somehow use that to get the absolute magnitude?

Edit: I also realllllllly need to know up to what distance in lightyears cepheids can be used to determine it's distance from the earth. On the order of 10^7 or 10^8 lightyears (I have two sources which are conflicting)?

[TheBalticSea]

I think they want you to assume the period is 3 days based on that graph (as a general rule, I find it useful to assume they want round numbers when they try to make graphs neat-looking like that). The period-luminosity chart i gave a link to is just an example; i think here they want you to use the period logs and absolute luminosities to create your own period-luminosity chart and then interpolate the absolute magnitude of the object in question based on that chart.
they provided apparent magnitude, right?

[crazy77]

Ohhhh that kinda makes sense. So basically you can put the numbers they give you into L1 and L2 on your calculator, find the equation that best fits (for this I got cubic, does that sound right? r^2 was 1 so I just used that one), put the equation into Y1, graph it, and do second Trace value, put in the log of 3 (.477) and get -2.77 for the absolute magnitude and then use the equation from there? Does that sound right? If you wanna try the problem yourself, the numbers they gave were:

Log of period: Absolute Magnitude:
1 -4.0
.68 -3.6
1.6 -6.3
.5 -2.9

And yes they did give the apparent magnitude (15.3)

[smartkid222]

Question:
A quasar's redshift is observed to be z= 3.65. Calculate the recessional velocity of the quasar, with respect to Earth?

Don't you just multiply z by c (the speed of light)?