<p>Yes.  It does mean that.  Double degenerate type 1a supernova would create supernovae of significantly different luminosity, and hence, in the words of Wikipedia, &quot;raise questions about the applicability of Type Ia supernovae as standard candles&quot;</p>

CC BY-SA 4.0
2024-04-26T05:29:21.400Z
2024-04-26T05:29:21.400Z
James K
<p>Type Ia supernovae are <em>not</em> standard candles in the sense that all such supernovae have the same luminosity. This is also the case for other standard candles such as Cepheids.</p>
<p>What we can say, is that type Ia supernovae appear to have a peak luminosity that can be calibrated using other, distance-independent, observable properties.</p>
<p>Indeed, the intrinsic luminosity of type Ia supernovae probably varies by as much as a factor of <strong>four</strong>. The underlying physical reasons for this are not yet fully understood but almost certainly include the type of progenitor (e.g. whether it was a double white dwarf), the detailed composition of the exploding white dwarf, and in particular, how much <span class="math-container">$^{56}$</span>Ni is synthesised in the explosion and ultimately powers the light curve.</p>
<p>The standard procedure at the moment is to apply empirical corrections to the &quot;raw&quot; absolute magnitude that depend on the width/shape of the light curve (the so-called &quot;stretch correction&quot;), the colour of the light curve and the stellar mass of the host galaxy - all of which have been found to correlate with the peak luminosity of the explosion. There are <em>many</em> papers discussing these corrections, but a good starting point is <a href="https://academic.oup.com/mnras/article/479/3/3996/5046725" rel="noreferrer">Hauret et al. (2018)</a>.</p>
<p>After applying these corrections, the dispersion of the corrected peak luminosity is about 0.1 magnitudes - turning these objects into &quot;standard candles&quot;.</p>
<p>It may be that a significant fraction of the type Ia supernovae are caused by merging white dwarfs. However, some theoretical studies (e.g., <a href="https://ui.adsabs.harvard.edu/abs/2022MNRAS.517.5260P/abstract" rel="noreferrer">Pakmor et al. 2024</a>) suggest that the explosion of the more massive white dwarf dominates the light curve shape and evolution and the smaller white dwarf may not explode at all. These then might fit neatly into the calibration scheme described above. Nevertheless, there are certain subclasses of type Ia supernovae, that exhibit peculiarities in their spectra or light curves, that are usually excluded from standard candle analyses. Whether these peculiarities stem from the nature of the binary companion or the spin and magnetic field of the exploding white dwarf is still debated (e.g. <a href="https://iopscience.iop.org/article/10.3847/1538-4357/acdd5d/meta" rel="noreferrer">Axen &amp; Nugent 2023</a>).</p>

CC BY-SA 4.0
2024-04-26T06:38:25.857Z
2024-04-26T07:13:41.280Z
2024-04-26T07:13:41.280Z
ProfRob
<p>I understand that the mass limit for a white dwarf is 1.4 solar masses, and therefore approaching it would cause a white dwarf (type Ia) supernova, and thus it is a standard candle. I understand this if the supernova is triggered by accreting mass.</p>
<p>However, if two white dwarfs merge and each has 1 solar mass, the mass at the time of the supernova would be 2 solar masses, thus producing more energy and the supernova would be brighter. Doesn't this mean that white dwarf supernovae produced via merger are not good standard candles?</p>

CC BY-SA 4.0
2024-04-26T08:15:52.530Z
If you look at the Pakmor et al. paper that ProfRob links to, it suggests that the sequence of events in the "merger scenario" is: WDs get very close together; strong tidal forces from more massive WD distort the less massive WD; material from outer parts of less massive WD start accreting onto more massive WD; more massive WD goes supernova while (part of) less massive WD is still separate.
Peter Erwin
CC BY-SA 4.0
2024-04-26T08:16:34.610Z
So it's not actually "two WDs merger completely, then supernova", as you might perhaps imagine.
Peter Erwin
<cosmology><supernova><white-dwarf><standard-candle>
White dwarf supernova luminosity
Cosmo
# White dwarf supernova luminosity

(Asked by: username_0 on 2024-04-26)

I understand that the mass limit for a white dwarf is 1.4 solar masses, and therefore approaching it would cause a white dwarf (type Ia) supernova, and thus it is a standard candle. I understand this if the supernova is triggered by accreting mass.

However, if two white dwarfs merge and each has 1 solar mass, the mass at the time of the supernova would be 2 solar masses, thus producing more energy and the supernova would be brighter. Doesn't this mean that white dwarf supernovae produced via merger are not good standard candles?

**Tags:** cosmology, supernova, white-dwarf, standard-candle

> **Comments:**
> **username_1**: If you look at the Pakmor et al. paper that username_3 links to, it suggests that the sequence of events in the "merger scenario" is: WDs get very close together; strong tidal forces from more massive WD distort the less massive WD; material from outer parts of less massive WD start accreting onto more massive WD; more massive WD goes supernova while (part of) less massive WD is still separate.  
> **username_1**: So it's not actually "two WDs merger completely, then supernova", as you might perhaps imagine.  

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## 2 answers

### Answer by username_3 on 2024-04-26. Score: 8 | Accepted answer

Type Ia supernovae are *not* standard candles in the sense that all such supernovae have the same luminosity. This is also the case for other standard candles such as Cepheids.

What we can say, is that type Ia supernovae appear to have a peak luminosity that can be calibrated using other, distance-independent, observable properties.

Indeed, the intrinsic luminosity of type Ia supernovae probably varies by as much as a factor of **four**. The underlying physical reasons for this are not yet fully understood but almost certainly include the type of progenitor (e.g. whether it was a double white dwarf), the detailed composition of the exploding white dwarf, and in particular, how much $^{56}$Ni is synthesised in the explosion and ultimately powers the light curve.

The standard procedure at the moment is to apply empirical corrections to the "raw" absolute magnitude that depend on the width/shape of the light curve (the so-called "stretch correction"), the colour of the light curve and the stellar mass of the host galaxy - all of which have been found to correlate with the peak luminosity of the explosion. There are *many* papers discussing these corrections, but a good starting point is [Hauret et al. (2018)](https://academic.oup.com/mnras/article/479/3/3996/5046725).

After applying these corrections, the dispersion of the corrected peak luminosity is about 0.1 magnitudes - turning these objects into "standard candles".

It may be that a significant fraction of the type Ia supernovae are caused by merging white dwarfs. However, some theoretical studies (e.g., [Pakmor et al. 2024](https://ui.adsabs.harvard.edu/abs/2022MNRAS.517.5260P/abstract)) suggest that the explosion of the more massive white dwarf dominates the light curve shape and evolution and the smaller white dwarf may not explode at all. These then might fit neatly into the calibration scheme described above. Nevertheless, there are certain subclasses of type Ia supernovae, that exhibit peculiarities in their spectra or light curves, that are usually excluded from standard candle analyses. Whether these peculiarities stem from the nature of the binary companion or the spin and magnetic field of the exploding white dwarf is still debated (e.g. [Axen & Nugent 2023](https://iopscience.iop.org/article/10.3847/1538-4357/acdd5d/meta)).


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### Answer by username_2 on 2024-04-26. Score: 3

Yes. It does mean that. Double degenerate type 1a supernova would create supernovae of significantly different luminosity, and hence, in the words of Wikipedia, "raise questions about the applicability of Type Ia supernovae as standard candles"


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