Turn on thread page Beta
    • Thread Starter
    Offline

    1
    ReputationRep:
    hi,
    This is from the jan 04 paper Q2, for astro aqa a.

    there's 2 stars, with the same apparent magnitude, one of spectral class A and the other of spectral class G.

    it's asks for which spectral class the smaller star belongs to? the answer is G.

    but i'm confused as i thought that as A is hotter, and the power output of each are the same, that surface area would be smaller for A? - using stefan's law.:confused:

    Any help appreciated.
    thanks
    Offline

    16
    ReputationRep:
    Temperature decreases along the horizontal axis of the Hertzsprung-Russell diagram.

    Stefans law P = stefans constant A T^4 = kAT^4 (or something like that...been a while since my astronomy module lol)

    P/kT^4 = A

    So area is inversely proportional to T^4

    So the star with the higher temperature is the smaller. So it should be class A unless we're both missing something...
    • Thread Starter
    Offline

    1
    ReputationRep:
    just double checked G is definitely the answer. ???
    Offline

    15
    ReputationRep:
    Basically, (I know this may be naff all help to you now, but let's give it a shot anyway), first consider what apparent magnitude actually is - the apparent brightness of a star from Earth. I'm slightly worried that they use apparent magnitude with absolute - if you ever get back, recheck that. I'm going to assume they're relatively similar distances from the Earth, as we don't have any other info to go on.

    With that in mind, if these two stars have the same apparent magnitude, then they both appear to have the same brightness.

    The classification of stars from hottest (and thus, brightest due to higher power output) to coolest is OBAFGKM . Hence, A will be hotter than G.

    Let's bring all this together. If A is hotter than G, then it has a higher power output per unit surface area. G has a lower power output per unit surface area.

    In order for these two to appear of the same brightness, then A has to have a larger surface area, so it's intensity is equal to that of the other star. Hence, the smaller star is G.
    Offline

    16
    ReputationRep:
    (Original post by chidona)
    Basically, (I know this may be naff all help to you now, but let's give it a shot anyway), first consider what apparent magnitude actually is - the apparent brightness of a star from Earth. I'm slightly worried that they use apparent magnitude with absolute - if you ever get back, recheck that. I'm going to assume they're relatively similar distances from the Earth, as we don't have any other info to go on.

    With that in mind, if these two stars have the same apparent magnitude, then they both appear to have the same brightness.

    The classification of stars from hottest (and thus, brightest due to higher power output) to coolest is OBAFGKM . Hence, A will be hotter than G.

    Let's bring all this together. If A is hotter than G, then it has a higher power output per unit surface area. G has a lower power output per unit surface area.

    In order for these two to appear of the same brightness, then A has to have a larger surface area, so it's intensity is equal to that of the other star. Hence, the smaller star is G.

    Seems I decided to completely ignore the whole apparent magnitude bit haha. My mistake. Good answer though!
    • Thread Starter
    Offline

    1
    ReputationRep:
    ohhhh i get it. so is power output a measure of temperature or brightness or both???

    Thanks for your replies guys.
    Offline

    15
    ReputationRep:
    Power output can be a measure of both, i believe, but someone feel free to contradict me. Usually in astro questions they'll just probe you about brightness, so don't worry =]

    Glad I could help.
    Offline

    0
    ReputationRep:
    No the mark scheme is wrong, i checked this question with my teacher
    As P = sigma A T^4 and P is constant (because Absolute magnitude is constant) then if T is larger A must be smaller
    Hense the hotter star, the A class star, is the smallest

    If you check the examiner's report it says that ' the best answers said that the smallest star was in spectral class A'
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: June 11, 2008

University open days

  • Heriot-Watt University
    School of Textiles and Design Undergraduate
    Fri, 16 Nov '18
  • University of Roehampton
    All departments Undergraduate
    Sat, 17 Nov '18
  • Edge Hill University
    Faculty of Health and Social Care Undergraduate
    Sat, 17 Nov '18
Poll
Have you ever experienced bullying?

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.