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Alex-RCHS wrote:What values do you normally use for the Hubble Constant?

A good test should give you the value you should use but I always default to 70 km/s/Mpc.

Alex-RCHS wrote:What values do you normally use for the Hubble Constant?

71 (km/sec)/MPc, unless given on test.

Thank you. Also, brightness is measured in watts per square meter, right? And that value is absolute (the same no matter where you measure it from)?

Alex-RCHS wrote:Thank you. Also, brightness is measured in watts per square meter, right? And that value is absolute (the same no matter where you measure it from)?

Yes and no. Otherwise the inverse square law of brightness wouldn't hold true. (Decreases with the distance squared). I believe they call this one apparent or observed brightness, but there might give you the true brightness (luminosity/surface area of star)

Alex-RCHS wrote:Thank you. Also, brightness is measured in watts per square meter, right? And that value is absolute (the same no matter where you measure it from)?

For Astronomy purposes, brightness tends to be used as apparent brightness, but can also be used as just luminosity (which is the total power emitted by the star).

Hmm. Okay, so luminosity is the total amount of light emitted by an object, in watts? Or watts per square meter?

And apparent/observed brightness is the total watts of light passing through one square meter at a certain distance away from the object?

If those are correct, what would be the name for the measure of the watts emitted by an object per unit of its surface area?

Alex-RCHS wrote:Hmm. Okay, so luminosity is the total amount of light emitted by an object, in watts? Or watts per square meter?

And apparent/observed brightness is the total watts of light passing through one square meter at a certain distance away from the object?

If those are correct, what would be the name for the measure of the watts emitted by an object per unit of its surface area?

1. Total, in watts
2. Yes, that’s how we measure it.
3. Flux (w/m^2)
Note: you can refer to the apparent brightness as flux through your telescope in w/m^2. But, flux in this context refers to the Stephan-Boltzmann law on the star’s surface.

These are great questions!

I need help!
This may be late but i'm a little (a lot) confused about this and my competition is on Friday

Here's an example problem:

A particular star has a parallax of
37.4 milliarcseconds and a proper motion of 173
milliarcseconds/year. Its peak wavelength is 415 nm, its apparent magnitude is 5.6, and its z
value is
What is the distance to this star in parsecs?
. What is the effective surface temperature of this star?
. What is the radial velocity of this star in km/s?
. What is the transverse velocity of this star in km/s?
. What is the
star’s absolute magnitude?
. What is the star’s luminosity in solar units?
. What is the star’s mass in solar units?
. What is the star’s radius in solar units?
(from the 2014 Penn. State test)

So how would you do... like pretty much any of this?
Thank you!

NePickers5 wrote:I need help!
This may be late but i'm a little (a lot) confused about this and my competition is on Friday

Here's an example problem:

A particular star has a parallax of
37.4 milliarcseconds and a proper motion of 173
milliarcseconds/year. Its peak wavelength is 415 nm, its apparent magnitude is 5.6, and its z
value is
What is the distance to this star in parsecs?
. What is the effective surface temperature of this star?
. What is the radial velocity of this star in km/s?
. What is the transverse velocity of this star in km/s?
. What is the
star’s absolute magnitude?
. What is the star’s luminosity in solar units?
. What is the star’s mass in solar units?
. What is the star’s radius in solar units?
(from the 2014 Penn. State test)

So how would you do... like pretty much any of this?
Thank you!

For astronomy, there's a lot of basic equations that you simply have to have in your binder. You don't need to memorize them (although I'm sure the best teams do) but you should be familiar enough with which variables they relate. Anyway...

What is the distance to this star in parsecs? There's a lot of ways to calculate distance, but the easiest is with parallax. The equation is d = 1/p, where d is distance in parsecs, and p is parallax in arcseconds (arcseconds are a unit of angle, like degrees or radians). Note: this is so convenient because the parsec is actually defined this way.

What is the effective surface temperature of this star? This uses wien's law, which relates the peak wavelength emitted to the temperature like this: (wavelength) * (temperature) = (a constant). The constant is .0029m.

Radial velocity is the linear velocity at which the star is moving away from the viewer. You can calculate this using the z value, which was mentioned but not given in the question (I think you left it out). Anyways, z * c = v where z is the redshift, c is the speed of light in a vacuum, and v is the radial velocity (also known as recessional velocity -- I still can't figure out if there is a serious difference between radial and recessional velocity, but I usually treat them as the same). Also, redshift, z, is defined as the change in the observed wavelength of light divided by the actual wavelength of light emitted by an object. If that doesn't make sense, you should look up redshift on wikipedia.

For transverse velocity, that is just the linear motion of the star perpendicular to the viewer. You are given the proper motion (in units angle/time) so to find the transverse velocity you just need to turn those units of angles into distance. You can use simple trigonometry to do this if you know the distance to the star (which you do, because of the first question).

For absolute magnitude, just use the distance modulus formula.

Luminosity - just convert absolute magnitude to luminosity.

Mass - not sure about this one actually. For many stars there are particular relationships between mass and other values. If this is a main-sequence star then there are many simplified relationships you could use.

Radius - use the luminosity and the radiant exitance (calculated via the stefan boltzmann law) to find the surface area, and then the surface area to find the radius.

If any part of that didn't make sense, I advise you to read the wikipedia article on it until it does. Or ask more questions!

Hope this helps!

Note: this is a good question, not only because it tests on many of the important math aspects of the event, but also because it follows a very college-style question structure, in which you are given all of the information upfront, and you have to figure out how to use each number to get the answer, as well as figure out which answers you need to answer subsequent parts of the question. This ability to discern what items you need where will be a good tool in your arsenal when you reach college homeworks.