How do we know the distance of a standard candle supernova?

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Offline thedoc

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David asked the Naked Scientists:
   I would like to know how astronomers/physicists know the distance of what is called ' a standard candle - super-nova', I believe  and use this as a means of measuring the distance of other stars or galaxies.

In other words, if they can measure what the distance is for a standard candle why can they not use this method for measuring the distance of other stars, galaxies etc etc?
What do you think?
« Last Edit: 04/02/2016 16:50:02 by _system »


Offline chiralSPO

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My understanding is that a standard candle is a very specific type of supernova that is always exactly the same brightness (hence the name "standard candle.") If we know how bright the supernova is and we can measure how bright it appears, we can calculate the distance using the formula dobs = da * sqrt(Ia/Iobs) where dobs is the distance between the star and the observer, Ia is the actual intensity of the light at a distance da (which is known) and Iobs is the intesity of the light we observe. For other objects like stars and galaxies, we don't know what I0 is, so we have two unknowns and only one equation, which cannot be solved.

Of course, this simple claculation assumes that there is nothing in the way (like a dust cloud that would absorb and scatter the light, or a large gravitational body that would bend the light), and it assumes that these supernovae are in fact standard.

There is a wealth of data that supports this methodology, but I do wonder sometimes if there are some bad assumptions in there that lead to misconceptions about the size/shape/growth of the observable universe...
« Last Edit: 04/02/2016 17:11:21 by chiralSPO »


Offline MurBob

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Type 1a supernova are standard candles because of how they become supernova's.  Type 1a supernova's only happen in binary star systems when a white dwarf star accumulates enough material from its companion star to reignite the fusion process.

When a white dwarf and another companion star orbit each other close enough, the white dwarf accumulates some of the other star's matter.  When enough matter has been accumulated, the white dwarf reignites the fusion process.  Because it's a white dwarf, a chain of events happens that not only fusses all the material at the same time, but sets off a chain of events that amplifies the bang.

The reason they are all the same brightness is because there is a sort of critical mass limit before they reignite and they all reach the about same critical mass when it happens.. therefor, they are all about the same brightness.

At least that's how I understand it.


Offline evan_au

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The brightness of a supernova is just one "rung" on a ladder of measurements on which we base our astronomical and cosmological distance measurements.

Unfortunately for astronomers (or fortunately for the rest of us), such supernovae are infrequent in our own galaxy, so we have to use additional methods.

It starts with parallax measurements of distance to nearby stars (which the Gaia satellite is busy refining, as we converse).

It also uses the oscillation period of pulsating stars, as other "rungs".


Offline Space Flow

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David asked the Naked Scientists:
I would like to know how astronomers/physicists know the distance of what is called ' a standard candle - super-nova', I believe  and use this as a means of measuring the distance of other stars or galaxies.
David as has been stated, Type 1a Supernovae are the type of Supernovae that are considered as standard candles.

Having said that, there is a not so advertised fact that, there are two theoretical models for these Supernovae.

1. One White Dwarf accreting gas of a companion star that has not made it to White Dwarf stage yet. If the White Dwarf's mass reaches a critical limit (about 1.38 Solar masses), it set's off a runaway Thermonuclear reaction that totally destroys the White Dwarf.
This is the scenario which is considered to be the most common and is the basis for the standard candle.
Because this explosion happens when a White Dwarf gets to a certain Mass, the energy released and the pattern of the explosion, as well as the spectroscopic signature, are all considered to be very similar. Similar enough to be considered as the same.
Knowing this, means we know the absolute luminosity of this type of Supernova, and by comparing the real luminosity to how bright they measure to us we can, using the inverse square law calculate the actual distance.

2.  The merger of two White Dwarfs.
This second Type Ia scenario, because it involves 2 White Dwarfs, has the same spectral signature as the first type. Also because it too results in a total thermonuclear destruction, it has the same timing in its rise and fall signature. But because it involves 2 White Dwarfs colliding, its power output and therefore the luminosity can be quite variable. This Type 1a Supernova would not make a very good standard candle.

Unfortunately we have no way to differentiate between the two.
We have basically made an assumption that the accretion Type 1a greatly outnumber the collision type.
There is some recently accumulated data, that seems to point towards the collision Type 1a, being more prevalent than the Accretion Type 1a.
This if true could cast doubt on the calibration of redshift to distance as that depends on the Type 1a as the reference. That would of course cast doubt on a lot of things including the age of the Universe.

Hope all that helps rather than confuses..
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