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Thread: Luminosity of the Milky Way galaxy?

  1. #1
    Join Date
    Feb 2010
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    Luminosity of the Milky Way galaxy?

    What is it? Does it vary with wavelength (or frequency)? What units can it be expressed in? How is it determined?

    Two earlier questions of mine, here, are good starting points: maggies, nanomaggies, janskies, etc, and Absolute magnitude of the Milky Way: what is it?

    Also helpful, the Wikipedia entries for Jansky and Luminosity (disambiguated, referring to astronomy).

    Some sentences, from WP:

    "In astronomy, luminosity is the total amount of energy emitted per unit of time by a star, galaxy, or other astronomical object.[1] As a term for energy emitted per unit time, luminosity is synonymous with power." OK, got it; luminosity is just power (energy per unit time)

    "Luminosity can also be given in terms of the astronomical magnitude system: the absolute bolometric magnitude (Mbol) of an object is a logarithmic measure of its total energy emission rate, while absolute magnitude is a logarithmic measure of the luminosity within some specific wavelength range or filter band." Wait, what? Absolute magnitude may (ignoring the "logarithmic measure") be something like "luminosity in the r-band"? Or "luminosity per unit wavelength", or even "luminosity per unit frequency"?? I'm confused.

    "The term luminosity is also used in relation to particular passbands such as a visual luminosity of K-band luminosity.[11] These are not generally luminosities in the strict sense of an absolute measure of radiated power, but absolute magnitudes defined for a given filter in a photometric system. Several different photometric systems exist. Some such as the UBV or Johnson system are defined against photometric standard stars, while others such as the AB system are defined in terms of a spectral flux density." (my bold) OK, that clears things up (a bit, anyway).

    From the link at the end of the quote above: "In spectroscopy, spectral flux density is the quantity that describes the rate at which energy is transferred by electromagnetic radiation through a real or virtual surface, per unit surface area and per unit wavelength (or, equivalently, per unit frequency)." Cool! I bet "Jansky" will appear soon ...

    "In SI units it is measured in [...] Jansky [...]" Bingo!

    (dropping the links) "The terms irradiance, radiant exitance, radiant emittance, and radiosity are closely related to spectral flux density. The terms used to describe spectral flux density vary between fields, sometimes including adjectives such as "electromagnetic" or "radiative", and sometimes dropping the word "density"." My word, this sure is confusing! Why can't astronomers settle on a single term?!?

    the luminosity in watts can be calculated from an absolute magnitude (although absolute magnitudes are often not measured relative to an absolute flux):

    L* = L0 x 10-0.4Mbol

    where L0 is the zero point luminosity 3.01281028 W
    OK, L* is not defined, but that's OK ... it's the luminosity of an object called *

    So, the answer to my question (what is the luminosity of the Milky Way galaxy?) is "you can work it out by plugging in the MW's Mbol in the above equation.

    But, but, but ... to get the r-band luminosity of the MW, is it simply plugging in Mr (the MW's estimated absolute magnitude in the r-band) instead of Mbol? Probably not (it seems too easy, and as the above clearly demonstrates, this topic is a minefield). If only because you'd need to know if Mr is in the AB magnitude system or not, right?

    And what about luminosity at a particular wavelength (or frequency), say 620 nm (sorta near the mid-point of the r-band)??

  2. #2
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    I would estimate that the bolometric luminosity of the Milky Way galaxy is on the order of 100 billion times that of the Sun. Some of that started as visible light from stars but has been absorbed by interstellar dust and is being reradiated as infrared. Published values of the absolute visual magnitude are closer to 10 billion times that of the Sun.

    It appears that we have an assortment of magnitude definitions to choose from, depending on which one is most useful for a particular task.

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