Astronomy C

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Re: Astronomy C

Postby PM2017 » January 1st, 2019, 3:18 pm

syo_astro wrote:As for images, yes, all the images (they're pretty too!). Hope this helps, please keep questions coming if that was confusing!

Just adding to this, you should also label each image (we include Telescope, Wavelength, Date if important, and for the non-obvious images, we include a description. By non-obvious I mean like the supernovae inside a DSO.)
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Re: Astronomy C

Postby GarethM » January 2nd, 2019, 7:51 pm

What reference value should be used for apparent/absolute visual magnitude? I can find luminosities for all wavelengths, but not just visible wavelengths.

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Re: Astronomy C

Postby Unome » January 3rd, 2019, 12:49 pm

GarethM wrote:What reference value should be used for apparent/absolute visual magnitude? I can find luminosities for all wavelengths, but not just visible wavelengths.

Your question is a little unclear. What sort of reference values are you looking for?
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Re: Astronomy C

Postby GarethM » January 3rd, 2019, 3:30 pm

I’ve seen questions that ask to convert absolute visual magnitude to visible luminosity measured in watts. I understand that a decrease in 5 magnitudes is a 100x increase in brightness, but I don’t understand what that increase is measured from, i.e. what value is defined as 0 on the magnitude scale. I was thinking I could measure relative to a star with a known absolute magnitude and visible luminosity, but I can’t find information on the visible luminosity of any stars. I’ve found info on bolometric magnitudes for all wavelengths, but nothing for just visible.

I’m basically wondering how to calculate visible luminosity in watts when given absolute visible magnitude.

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Re: Astronomy C

Postby Unome » January 4th, 2019, 4:44 am

GarethM wrote:I’ve seen questions that ask to convert absolute visual magnitude to visible luminosity measured in watts. I understand that a decrease in 5 magnitudes is a 100x increase in brightness, but I don’t understand what that increase is measured from, i.e. what value is defined as 0 on the magnitude scale. I was thinking I could measure relative to a star with a known absolute magnitude and visible luminosity, but I can’t find information on the visible luminosity of any stars. I’ve found info on bolometric magnitudes for all wavelengths, but nothing for just visible.

I’m basically wondering how to calculate visible luminosity in watts when given absolute visible magnitude.

My typical method is to take the Sun's absolute magnitude of around 4.85 and use its known luminosity in watts to convert.
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Re: Astronomy C

Postby PM2017 » January 4th, 2019, 11:51 pm

Unome wrote:
GarethM wrote:I’ve seen questions that ask to convert absolute visual magnitude to visible luminosity measured in watts. I understand that a decrease in 5 magnitudes is a 100x increase in brightness, but I don’t understand what that increase is measured from, i.e. what value is defined as 0 on the magnitude scale. I was thinking I could measure relative to a star with a known absolute magnitude and visible luminosity, but I can’t find information on the visible luminosity of any stars. I’ve found info on bolometric magnitudes for all wavelengths, but nothing for just visible.

I’m basically wondering how to calculate visible luminosity in watts when given absolute visible magnitude.

My typical method is to take the Sun's absolute magnitude of around 4.85 and use its known luminosity in watts to convert.

I do this as well.
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Re: Astronomy C

Postby jz123sst » January 11th, 2019, 1:55 pm

Does anyone know if Astronomy will still be an event 2019-2020 (next year)? If so, what topics will it cover?

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Re: Astronomy C

Postby pb5754[] » January 11th, 2019, 3:53 pm

jz123sst wrote:Does anyone know if Astronomy will still be an event 2019-2020 (next year)? If so, what topics will it cover?

It definitely will be an event next year, and I'm not sure but I think the topic will be variable stars or star/planet formation.

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Re: Astronomy C

Postby jz123sst » January 11th, 2019, 4:38 pm

So stellar evolution is a topic every year?

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Re: Astronomy C

Postby pb5754[] » January 11th, 2019, 4:45 pm

jz123sst wrote:So stellar evolution is a topic every year?

Yes... like there is a 99.9% chance I would say.

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Re: Astronomy C

Postby syo_astro » January 11th, 2019, 6:27 pm

pb5754[] wrote:
jz123sst wrote:Does anyone know if Astronomy will still be an event 2019-2020 (next year)? If so, what topics will it cover?

It definitely will be an event next year, and I'm not sure but I think the topic will be variable stars or star/planet formation.


Disclaimer: not official, but my personal experience from the past about 7 to 9 yrs or so (but still not official!!)

The exact topic changes every single year. There are some small patterns, but it really can change a lot. Also, "stellar evolution is always there" is pretty much true nowadays (it could change randomly, but at least in "recent" times it's been true...see this year: "stellar evolution" still around with *galaxies* >.>). But like I said the sub-topic is important / less predictable (though, things like light curves, spectra, images, DSOs, etc are always important...that should be clear if you have *any* two years of rules).
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Re: Astronomy C

Postby SciolyHarsh » January 13th, 2019, 8:14 am

One of the tests I took asked me to use luminosity in lumens to find the brightness of a star. Is there a way to convert lumens to watts, because I can't see another way to solve this(we were given a distance, so I tried using L=I/(4pir^2)
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Re: Astronomy C

Postby PM2017 » January 13th, 2019, 8:59 am

SciolyHarsh wrote:One of the tests I took asked me to use luminosity in lumens to find the brightness of a star. Is there a way to convert lumens to watts, because I can't see another way to solve this(we were given a distance, so I tried using L=I/(4pir^2)

I have never heard of lumens being used in an astro test. Could you send a screencap of that specific question? The dimensions for luminosity are Watts, not Lumens, so the validity of the question seems somewhat dubious.
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Re: Astronomy C

Postby dkarkada » Yesterday, 12:31 am

Hi everyone!

I've uploaded the MIT astro exam/key here, as well as some statistics from the competition. Be warned: it is really long and hard (Adi1008 and I had a bit too much fun with it :P). But hopefully everyone can learn something from it! To that effect, I've also uploaded a walkthrough of the free response questions, which (I hope) emphasizes how you should reason about the problems, and provides some resources for further reading. Keep checking back over the course of this week, as the walkthrough document will be edited to include more detailed solutions of Adi1008's questions as well.

We hope that you find this useful! As always, feel free to contact us if you have any questions.


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