Astronomy C

[Unome]

Here's my question:



(a) What do the colored lines represent?
(b) Rank A, B, C, and D in terms of mass
(c) What is the x-coordinate of the point labeled 2015, in Kelvin?

Going to move my question to this thread: what is this?

Also, two unrelated questions:

1) How do I convert between luminosity and magnitude?
2) How do I use spectroscopic parallax?

[Magikarpmaster629]

Update on Henize 2-428: soinc.org added a clarification that it is indeed 2-428 and not 2-248 as it says in the rules.

1) How do I convert between luminosity and magnitude?
2) How do I use spectroscopic parallax?

1)
where L is luminosity in solar luminosities and M is absolute magnitude.
2) Spectroscopic parallax is basically an application of the distance modulus. More technically, it's where you use tables to find the luminosity of a star you know the spectral type of; then by finding the apparent magnitude you can use the distance modulus and find its distance. I haven't ever seen it used, but it could happen.

Now for my own question: is there a clean conversion between Julian and Gregorian date, or do I need a program for that?

[Unome]

Update on Henize 2-428: soinc.org added a clarification that it is indeed 2-428 and not 2-248 as it says in the rules.

1) How do I convert between luminosity and magnitude?
2) How do I use spectroscopic parallax?

1)
where L is luminosity in solar luminosities and M is absolute magnitude.
2) Spectroscopic parallax is basically an application of the distance modulus. More technically, it's where you use tables to find the luminosity of a star you know the spectral type of; then by finding the apparent magnitude you can use the distance modulus and find its distance. I haven't ever seen it used, but it could happen.

Now for my own question: is there a clean conversion between Julian and Gregorian date, or do I need a program for that?

Thanks.

For your question: I usually just count the number of centuries over and subtract the 400 multiples, then add (or subtract) this from the other one.

[Adi1008]

Here's my question:



(a) What do the colored lines represent?
(b) Rank A, B, C, and D in terms of mass
(c) What is the x-coordinate of the point labeled 2015, in Kelvin?

Going to move my question to this thread: what is this?
(...)

This is a plot of surface gravity (on the y axis) vs. temperature (on the x axis) of post AGB stars on their way to becoming the central star of a planetary nebula. The lines show different evolutionary tracks over time for post AGB stars of different masses.

More massive stars are (in general) hotter, which is why the more massive stars are on the left, and vice versa

By knowing the surface gravity, it also gives us properties like the radius (size) and luminosity.

You might be wondering what the g/(cm/s^2) stuff is. Initially, it may look like grams/(cm/s^2), but the argument of a logarithm should be dimensionless - that is, the units should cancel out in order for the logarithm to make sense. When it says "g", they don't mean g for "gram", they mean "g" for surface gravity, like g = -9.8 m/s^2 for Earth. It's hard to know that they're talking about that type of "g" though, but you can still figure it out. Since the argument of the logarithm has to be dimensionless, you know that whatever "g" is must have the same dimensions as "cm/s^2", which is just acceleration. Personally, when I see "g" and the units for acceleration, it's clear as to what it's trying to say. Additionally, log(g) is a common term in astronomy (I think, at least; Wikipedia says surface gravity is often expressed as log(g): https://www.wikiwand.com/en/Surface_gravity). You'l notice that's why plots have things like log(T_effective/K); the K is to get rid of the unit on the temperature.

Magikarpmaster asked about this in the question marathon thread too; I'll post some more stuff there too

[bhavjain]

Do Type II Cepheids have a longer period than Type I?

[Magikarpmaster629]

Do Type II Cepheids have a longer period than Type I?

I don't think they can, although this graph may not show everything.

[bhavjain]

Here's my question:



(a) What do the colored lines represent?
(b) Rank A, B, C, and D in terms of mass
(c) What is the x-coordinate of the point labeled 2015, in Kelvin?

Going to move my question to this thread: what is this?
(...)

This is a plot of surface gravity (on the y axis) vs. temperature (on the x axis) of post AGB stars on their way to becoming the central star of a planetary nebula. The lines show different evolutionary tracks over time for post AGB stars of different masses.

More massive stars are (in general) hotter, which is why the more massive stars are on the left, and vice versa

By knowing the surface gravity, it also gives us properties like the radius (size) and luminosity.

You might be wondering what the g/(cm/s^2) stuff is. Initially, it may look like grams/(cm/s^2), but the argument of a logarithm should be dimensionless - that is, the units should cancel out in order for the logarithm to make sense. When it says "g", they don't mean g for "gram", they mean "g" for surface gravity, like g = -9.8 m/s^2 for Earth. It's hard to know that they're talking about that type of "g" though, but you can still figure it out. Since the argument of the logarithm has to be dimensionless, you know that whatever "g" is must have the same dimensions as "cm/s^2", which is just acceleration. Personally, when I see "g" and the units for acceleration, it's clear as to what it's trying to say. Additionally, log(g) is a common term in astronomy (I think, at least; Wikipedia says surface gravity is often expressed as log(g): https://www.wikiwand.com/en/Surface_gravity). You'l notice that's why plots have things like log(T_effective/K); the K is to get rid of the unit on the temperature.

Magikarpmaster asked about this in the question marathon thread too; I'll post some more stuff there too

How do you get the temperature of the x-coordinate? Sorry, new to Astro

[Adi1008]

How do you get the temperature of the x-coordinate? Sorry, new to Astro

The x coordinate shows what log(T/K)* is equal to. For example, if the x coordinate is 10, then that means that log(T/K) = 10, so T = 10^10 Kelvin (that's really hot!).

Applying this to the plot... the point has an x coordinate of around 4.7, so that means log(T/K) = 4.7. Recall that just saying "log" means the base is 10, so that means the temperature is 10^4.7 Kelvin. Plugging this into a calculator gives you about 50118 Kelvin, which is the answer.

*Remember that the T/K business just means the natural log of the temperature, in Kelvin. The "/K" is to get rid of the unit (Kelvin), but I've seen it written without the "/K" as well. If it makes it easier to think about, just think of it as taking the natural log of a number (so it would have no units)

Hope this helps!

[EastStroudsburg13]

Generally, I don't see it written all within the same paretheses like that, because of the confusion it causes, but in this case at least the variable is italicized and the units are not. Remember when writing T/K, it's not actually a fraction. It's more like T {K}.

[bhavjain]

Is it possible to derive the distance modulus formula from the inverse square law and other laws?