## Astronomy C

PM2017
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### Re: Astronomy C

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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GarethM
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### Re: Astronomy C

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

Unome
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### Re: Astronomy C

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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GarethM
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### Re: Astronomy C

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.

Unome
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### Re: Astronomy C

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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PM2017
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### Re: Astronomy C

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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jz123sst
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### Re: Astronomy C

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

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.

jz123sst
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### Re: Astronomy C

So stellar evolution is a topic every year?

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### Re: Astronomy C

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

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

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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PM2017
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### Re: Astronomy C

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

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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### Re: Astronomy C

Do you know of any good resources to learn how to use JS9? It's seems confusing and I'm not sure how to answer questions about it.
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