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PM2017 wrote:

Name wrote:1. A star with the same radius and twice the temperature of our sun is found. What is its luminosity (in solar luminosity)?
2. The parallax of a type 1a supernova was found to be .05 milliarcsec. What its distance (in parsecs)?
3. What is the supernovas apparent magnitude?
4. The apparent magnitude of the star in part 1 and the supernova in part 2/3 is the same. What is the distance of the star?

attempt (Click to Show Hidden Text)

1. 8  (stefan boltzmann)
2. 20,000 (1000/parallax)
3. -2.8 (distance modulus; abs mag is -19.3)
4. really close, or 0.859 parsecs (Abs mag = 2.53, because the 2.5th root of 8 is ~2.30 4.83 - that is 2.53; apparent magnitude is -2.83)

Sorry, I was being lazy and did all this on a computer and no paper or pencil, and I just felt lazy, so lazy in fact that I decided to make this sentence a run on sentence.

Steuben42 wrote:It seems to me that the thread's dried up, so I guess I'll post a question that I think is new.

A star has been observed to have a radius of 3.42 solar radii and a temperature of 6141 K. What is the star's luminosity?

L/Lsun = (R/Rsun)^2(T/Tsun)^4
L = (3.42)^2(6141K/5772K)^4
L = 15.0 solar luminosities

pb5754[] wrote:

Steuben42 wrote:It seems to me that the thread's dried up, so I guess I'll post a question that I think is new.

A star has been observed to have a radius of 3.42 solar radii and a temperature of 6141 K. What is the star's luminosity?

attempt (Click to Show Hidden Text)

L/Lsun = (R/Rsun)^2(T/Tsun)^4
L = (3.42)^2(6141K/5772K)^4
L = 15.0 solar luminosities

pb5754[] wrote:1. What is the gravitational potential energy of a star with twice the mass of the sun and 3 times the radius?
2. What is the total kinetic energy of the star?

Hint: The star is in a virialized state.

SciolyHarsh wrote:

pb5754[] wrote:1. What is the gravitational potential energy of a star with twice the mass of the sun and 3 times the radius?
2. What is the total kinetic energy of the star?

Hint: The star is in a virialized state.

Potential Energy= -3.032x10^41 J
Kinetic Energy=1.52x10^41 J

I'm new to Astro math, so if I get this wrong, could you please show me the work in order to get the correct answer?

SciolyHarsh wrote:Given that the apparent magnitude of a type 1a supernova is 5.75, and the redshift is 0.67, find the age of the universe, assuming the Hubble constant is 70 (km/s)/Mpc.

The age of the universe I believe is T = 1/H.
This gives us T = (0.014286 Mpc*s/km)(3.086*10^19 km/Mpc) = 4.41*10^17 s = 14.0 billion years.

pb5754[] wrote:

SciolyHarsh wrote:Given that the apparent magnitude of a type 1a supernova is 5.75, and the redshift is 0.67, find the age of the universe, assuming the Hubble constant is 70 (km/s)/Mpc.

attempt? (Click to Show Hidden Text)

The age of the universe I believe is T = 1/H.
This gives us T = (0.014286 Mpc*s/km)(3.086*10^19 km/Mpc) = 4.41*10^17 s = 14.0 billion years.

Did you really need the apparent magnitude and redshift? or am I doing this wrong?

I think the question accidentally added the Hubble constant. It should have been an attempt to derive it on your own.