Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Mass Luminosity relation doesn't hold true when applied to actual data

  1. May 13, 2013 #1
    I see there have been many postings about this topic in this forum.

    The formula for this relation is Luminosity = Mass^3.5
    Taking logs of both sides we get log (lum) = 3.5 * log (mass)
    and using a little algebra we find that the exponent (3.5) should equal log (lum) ÷ log (mass)

    I have selected stars from a table located here: http://www.essex1.com/people/speer/main.html [Broken]
    and I have computed what the exponent value should be in each case.
    STAR   Lum   Mass   EXP
    Orionis C 30,000.00   18.00  3.566653
    Becrux   16,000.00   16.00   3.491446
    Spica   8,300.00   10.50   3.837759
    Achernar   750.00   5.40   3.925568
    Rigel   130.00   3.50   3.885439
    Sirius A   63.00   2.60   4.336039
    Fomalhaut   40.00   2.20   4.678604
    Altair   24.00   1.90   4.951367
    Polaris A   9.00   1.60   4.674910
    Eta Scorpii   6.30   1.50   4.539354
    Procyon A   4.00   1.35   4.619371
    Alpha Centauri A   1.45   1.08   4.827943
    The Sun   1.00   1.00
    Mu Cassiopeiae   0.70   0.95   6.953637
    Tau Ceti   0.44   0.85   5.051600
    Pollux   0.36   0.83   5.483033
    Epsilon Eridani   0.28   0.78   5.123395
    Alpha Centauri B   0.18   0.68   4.446371
    Lalande 21185   0.03   0.33   3.162872
    Ross 128   0.0005   0.20   4.722706
    Wolf 359   0.0002   0.10   3.698970

    No matter whether we concentrate on low mass or high mass stars, we see that the exponent value varies quite a bit .
    Does anyone know why there is so much variance in the mass luminosity relation?
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. May 13, 2013 #2


    User Avatar
    Science Advisor
    Gold Member

    A few things you should know about the mass-luminosity formula:

    The exponent value is an approximation. It varies from ~2.5 for stars of masses lower than the Sun's through ~4 to ~3 for masses much larger than that. 3.5 is just an average, a useful "rule of thumb".

    The reason for the variation is the changing way the energy is transported in the star(i.e., is there convection or not), and the contribution of radiation pressure in mainitaining hydrostatic equilibrium.

    There are other factors affecting the exponent value, such as metallicity and age

    The relation is an empirical one, derived from observations of binary(and multiple) systems, as these are (usually)the only ones for which mass can be determined accurately.

    Masses of single main-sequence stars are usually inferred from the relation.

    Additionally, the Mass-Luminosity relation concerns only main-sequence stars(stellar clasification includes the Roman "V" number). It is not applicable to e.g., giants, like Beta Crucis, Rigel, or Spica.

    Furthermore, the table you're using is very inaccurate. Most of the masses and luminosities do not match other sources I've found, including the arxiv article linked above(table 1). Looking up the sources for mass and luminosity of specific stars on wikipedia should give you more reliable data.
    For example, Altair has been measured to have the luminosity of ~10.6 times solar, not 24, and the mass is found to be ~1.79 solar masses, not 1.9
    (also an example of a star whose mass has been found by a different method)

    Lastly, I recommend this resouces if you want to get yourself acquainted with the guts of stellar astrophysics:

    Explore subjects in "astro 501" and "534".
  4. May 14, 2013 #3
    Well, thanks for the quick reply and very informative answer.
    I know the Internet is not 100% error-free but you would think that data posted to a scientific web page would be more reliable and accurate than someone's personal Facebook account.
    I never knew that a scientific formula could be so approximate as the mass-luminosity relation.
    I am currently working on a new web page that will explain the relationship between stellar mass and luminosity, absolute magnitude, temperature, spectral class and longevity.
    Thanks again.
  5. May 14, 2013 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Scientific data is constantly subject to revision. New measurement techniques are being developed which change older data. In astronomical observations of distant objects, a lot of the data may be only an estimate of the true values, since direct measurement is impossible.
  6. May 14, 2013 #5


    User Avatar
    Science Advisor
    Gold Member

    @SteamKing: makes you wonder, though, how old is the data if you see Pollux listed with such attributes as in that table. It doesn't even agree with the nice big picture above it.

    I've got nothing against Mr.Speer, but a highschool chemitsry teacher's personal webpage hardly qualifies as a scientific source. If only he had included the sources for his table, you'd be able to track it down and see at a glance if it isn't horribly outdated.

    The bottom line is, always look for sources.
    Last edited: May 14, 2013
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook