Apparent v Absolute Magnitude

[Nick P]

On the 'starry night' software stars are designated with two magnitude values, apparent and absolute.

I think 'apparent' is the visual magnitude (i.e. what we see) however I have also noticed that some absolute magnitude values are marked as 'visual'. As some of the differences between the two values can be quite large it can be quite confusing to know what to look for (a bright star or a dim one).

Can anyone explain the difference between these two terms?

[Demonperformer]

Absolute is calculated as how it would look at ... i think the distance is 10 parsecs

Apparent is how it looks from here

[barkis]

Absolute magnitude, is how bright an object would appear, if it were 10 parsecs distant from us. Or 33 light years.

Apparent magnitude, is an objects intrinsic value.

Ron.

[Ben Ritchie]

Apparent magnitude is a measure of how bright a star looks to an observer on Earth. Absolute magnitude is the brightness the star would have if it was at a distance of 10 parsecs (a bit over 30 light-years).

Some stars - e.g. Vega - appear bright because they're close. Vega has an apparent magnitude of 0.03, but an absolute magnitude of 0.58 because it's not actually a very luminous star. Others, such as Deneb, are hugely luminous but very far away; in that case the apparent magnitude is 1.25 (so fainter than Vega) but the absolute magnitude is -6.95, because it's a very luminous supergiant about 1400 light years from Earth. If it was as close as Vega it would far outshine everything other than the full moon.

The most luminous stars have absolute magnitudes around -10, and would rival the full moon even from a distance of 30 light years

[Nick P]

Thanks for the great explanations - clear as a bell now!

Best

Nick

[ollypenrice]

Actually I think one part of your question has been overlooked. Absolute Magnitude needs to have the wavelengths over which it has been measured defined. When you see it defined as absolute magnitude (visual) that means that its magnitude has been measured in the visible (visual) spectrum. However, some very hot stars are extremely bright just beyond the visible spectrum in the ultra violet. This short wavelength light would not contribute to the measurement in a 'visual' absolute mag so there is also Absolute Magnitude (bolometric). In this case the whole spectrum is measured. When you switch between the two measures some hot stars swap places on the mag scale, the hottest ones not peaking in visual and only showing their position at the top of the tree in bolometric.

I'm an amateur so a professional confirmation or rebuttal would be a good thing.

Olly

[Ben Ritchie]

I'm an amateur so a professional confirmation or rebuttal would be a good thing.

Well it varies a bit, but typically (absolute) magnitudes would be assumed to be visual unless stated otherwise, luminosities are bolometric (and usually given in terms of solar luminosity).

However, some very hot stars are extremely bright just beyond the visible spectrum in the ultra violet. This short wavelength light would not contribute to the measurement in a 'visual' absolute mag so there is also Absolute Magnitude (bolometric). In this case the whole spectrum is measured. When you switch between the two measures some hot stars swap places on the mag scale, the hottest ones not peaking in visual and only showing their position at the top of the tree in bolometric.

For example, if you take two very luminous stars: Cyg OB2#12 and HD93129A, the former is generally stated as having a higher absolute magnitude than the latter. However, Cyg OB2#12 is a B5Ia+ hypergiant, while HD93129A is a much hotter O2Iaf* supergiant, and when you include the UV flux it is more luminous across the whole spectrum.

And it's not just hot stars, cool stars also radiate most of their energy outside the visual spectrum.

[ollypenrice]

Thanks Ben.

Olly

[Rui]

As a remark, as absolute magnitude is a measure of the star output from a distance of 10 parsec (32,6l.y.), any star closer to us shows a higher visual magnitude than it's absolute magnitude.

Examples of this are Sirius, Alfa Centaurus or Vega.