<p>The other answer is right but it's interesting to see how the half a billion supernovae claim compares to what NASA has found.</p>
<p>According to the link, &quot;<a href="https://science.nasa.gov/missions/webb/nasas-webb-opens-new-window-on-supernova-science/#h-image-a-jades-deep-field-annotated" rel="nofollow noreferrer">the JADES Transient Survey Sample team uncovered about 80 supernovae in a patch of sky only about the thickness of a grain of rice held at arm’s length</a>&quot;. That's not a rigorous measure of the field of view but assuming a 1mm x 1mm field at 1m distance, it covers 1e-6 stereo-radians. The whole sky is (4·pi)=12.56 stereo-radians. Therefore if that patch is a representative sample of the sky and the same survey could be done for the whole sky, we would expect to get 80·(4·pi)/(.001·.001)=1,005,309,649 supernovae, which is a more than half a billion but in the same order of magnitude. Taking a larger grain of rice or a shorter arm yields different numbers of the same order. A 1.5mm grain of rice and an 1m arm would lower the estimate to  446,804,289 supernovae for the whole sky.</p>
<p>Therefore, we can accept the half a billion supernovae as a reasonable ballpark estimate.</p>
<p>And just for completeness, 80 observations in a sample yields a 95% confidence interval (63.43502, 99.56692) for rice grain sized patches of sky using poisson.test in R. However the main uncertainty in this question is the size of the field of view.</p>

CC BY-SA 4.0
2024-06-26T17:10:24.433Z
But interestingly, agrees with my 0.3 billion figure. Which is anpther way of saying that the observed SN rate is consistent with the known star forming rate of the universe and a universal IMF. However, my number does NOT account for observational incompleteness, which there surely must be. But I think JADES is sensitive enough to get the peak in the SN rate at $1<z<3$.
ProfRob
CC BY-SA 4.0
2024-06-26T18:33:17.670Z
@PM2Ring - Thank you. I was misremembering the formula for volume instead of area. That's the reason I had to use such a tiny grain of rice to meet the claim.
Pere
CC BY-SA 4.0
2024-06-26T17:00:07.507Z
2024-06-27T10:55:42.013Z
2024-06-27T10:55:42.013Z
Pere
<p>It depends what you mean by &quot;exploding right now&quot;. If you mean &quot;at this cosmic epoch&quot;, then that estimate looks too large by an order of magnitude. If you mean &quot;what you could observe with an unlimitedly powerful telescope over the whole sky&quot;, then half a billion is probably about right.</p>
<h2>Details</h2>
<p>There is an excellent estimate that there have been <span class="math-container">$3\times 10^{19}$</span> core-collapse supernovae that have occurred in the universe, which you can find in <a href="https://astronomy.stackexchange.com/questions/46769/can-we-estimate-the-number-of-stars-which-have-died-in-the-observable-universe">this answer</a>. The light curve of a core-collapse supernova spends about 50 days within 2 mag of its maximum.</p>
<p>If we now assume that supernovae occur evenly in time over, say, 13 billion years, then there are 0.3 billion supernovae occurring in any 50-day period. Or, to put it another way, given that the decay phase of the light curve is much longer than the rise phase, there are about 0.3 billion supernovae that have occurred within the last 50 days.</p>
<p>The assumption that supernovae are occurring uniformly in time is a really poor one. The formation rate of massive stars was much larger (by a factor of 10) between redshifts of 1–3, corresponding to about 8–12 billion years ago. This is when most of the supernovae occurred and is the population being measured by JWST in the cited press release.</p>
<p>Thus, if you want to know how many supernovae are exploding &quot;right now&quot; (at this cosmic epoch), then it is probably about 30 million. On the other hand, if you have a very powerful telescope that can see to redshifts of about <span class="math-container">$z \sim 8$</span> (roughly when star formation begins), then the time dependence of the supernova rate doesn't matter, and we can say that there are around 300 million taking place that <em>that we can observe now</em>. But the majority of those supernovae will be at <span class="math-container">$1 &lt; z &lt; 3$</span>.</p>
<p>Does this tally with a crude estimate of the numbers of stars in the universe? In a standard distribution of birth masses, about 1% of stars that are ever born will have <span class="math-container">$M&gt;8M_\odot$</span> and are candidates to end their lives as supernovae. Their very short lives (<span class="math-container">$\ll$</span> a billion years) means that almost all the massive stars ever born have died
- only about one in ten thousand is alive now, given the likely star forming history. It is uncertain what fraction of massive stars actually do explode at the ends of their lives, as opposed to being, say, failed supernovae, but assuming it is all of them and using 1% for the high-mass fraction, the estimate of <span class="math-container">$3\times 10^{19}$</span> high-mass stars being born translates to a total number of stars in the (observable) universe of <span class="math-container">$\sim 3 \times 10^{21}$</span>.</p>

CC BY-SA 4.0
2024-06-24T21:18:26.067Z
>"1% of stars that are ever born will have M>8M⊙ and are candidates to end their lives as supernovae" ==[ I didn't know it was so low! But are no large blues being born at the top of the HR anymore? What about Rigel and Sirius? They're pop 1, right?
Miss Understands
CC BY-SA 4.0
2024-06-24T22:11:47.110Z
@Miss_Understands yes, massive stars are being born now. Just not as frequently as 10 Gyr ago.  Rigel and Sirius (Sirius is a relatively low-mass star and will not go supernova) are Pop 1.
ProfRob
CC BY-SA 4.0
2024-06-24T22:50:21.957Z
what's the ratio of super blue star creation now compared to the first GY?
Miss Understands
CC BY-SA 4.0
2024-06-24T23:42:28.203Z
@Miss_Understands There is a plot referenced in https://astronomy.stackexchange.com/questions/46769/can-we-estimate-the-number-of-stars-which-have-died-in-the-observable-universe
ProfRob
CC BY-SA 4.0
2024-06-25T00:22:36.543Z
thanx, perfesser! It looks like two orders of magnitude, but that's the strangest vertical axis I've ever seen, even in astronomy. Log of solar masses per year divided by the cube root of the distance!
Miss Understands
CC BY-SA 4.0
2024-06-25T17:18:59.500Z
@Miss_Understands it's a cube, not a cube root. Mass per year per volume. Mpc is just any old unit of length in this case, not distance from Earth.
hobbs
CC BY-SA 4.0
2024-06-25T18:39:09.687Z
>Mass per year per volume.  ==[ Yeah, that would be it. And whaddya know; it's simple! Thanx!
Miss Understands
CC BY-SA 4.0
2024-06-26T04:11:03.677Z
@ProfRob *"Just not as frequently as 10 Gyr ago"* Does this mean that it would be more correct to say *"**almost** all the massive stars ever born have died"*?
vsz
CC BY-SA 4.0
2024-06-24T06:34:59.287Z
2024-06-26T07:47:48.607Z
2024-06-26T07:47:48.607Z
ProfRob
<p>By comparing 2 deep fields, <a href="https://science.nasa.gov/missions/webb/nasas-webb-opens-new-window-on-supernova-science/#h-image-a-jades-deep-field-annotated" rel="noreferrer">NASA found</a> 80 active supernovas in an area of the sky that's a rice grain at arm's length.</p>
<p><a href="https://www.youtube.com/watch?v=K1J-QBl15lE" rel="noreferrer">This guy on YouTube says</a> it means there are HALF A BILLION supernovas exploding right now.</p>
<p>Stars live billions of years and are visible as supernovae for, what, a week? My real question (that I couldn't fit in a title) is, what does this say about the total population Of stars in the universe?</p>
<p>Science says there are 200 billion trillion stars.</p>
<p>Are we WAAAAY off?</p>

<supernova>
Are there really half-a billion visible supernovae exploding all the time?
Miss Understands
# Are there really half-a billion visible supernovae exploding all the time?

(Asked by: username_0 on 2024-06-24)

By comparing 2 deep fields, [NASA found](https://science.nasa.gov/missions/webb/nasas-webb-opens-new-window-on-supernova-science/#h-image-a-jades-deep-field-annotated) 80 active supernovas in an area of the sky that's a rice grain at arm's length.

[This guy on YouTube says](https://www.youtube.com/watch?v=K1J-QBl15lE) it means there are HALF A BILLION supernovas exploding right now.

Stars live billions of years and are visible as supernovae for, what, a week? My real question (that I couldn't fit in a title) is, what does this say about the total population Of stars in the universe?

Science says there are 200 billion trillion stars.

Are we WAAAAY off?

**Tags:** supernova


---

## 2 answers

### Answer by username_2 on 2024-06-24. Score: 35 | Accepted answer

It depends what you mean by "exploding right now". If you mean "at this cosmic epoch", then that estimate looks too large by an order of magnitude. If you mean "what you could observe with an unlimitedly powerful telescope over the whole sky", then half a billion is probably about right.

## Details

There is an excellent estimate that there have been $3\times 10^{19}$ core-collapse supernovae that have occurred in the universe, which you can find in [this answer](https://astronomy.stackexchange.com/questions/46769/can-we-estimate-the-number-of-stars-which-have-died-in-the-observable-universe). The light curve of a core-collapse supernova spends about 50 days within 2 mag of its maximum.

If we now assume that supernovae occur evenly in time over, say, 13 billion years, then there are 0.3 billion supernovae occurring in any 50-day period. Or, to put it another way, given that the decay phase of the light curve is much longer than the rise phase, there are about 0.3 billion supernovae that have occurred within the last 50 days.

The assumption that supernovae are occurring uniformly in time is a really poor one. The formation rate of massive stars was much larger (by a factor of 10) between redshifts of 1–3, corresponding to about 8–12 billion years ago. This is when most of the supernovae occurred and is the population being measured by JWST in the cited press release.

Thus, if you want to know how many supernovae are exploding "right now" (at this cosmic epoch), then it is probably about 30 million. On the other hand, if you have a very powerful telescope that can see to redshifts of about $z \sim 8$ (roughly when star formation begins), then the time dependence of the supernova rate doesn't matter, and we can say that there are around 300 million taking place that *that we can observe now*. But the majority of those supernovae will be at $1 < z < 3$.

Does this tally with a crude estimate of the numbers of stars in the universe? In a standard distribution of birth masses, about 1% of stars that are ever born will have $M>8M\_\odot$ and are candidates to end their lives as supernovae. Their very short lives ($\ll$ a billion years) means that almost all the massive stars ever born have died
- only about one in ten thousand is alive now, given the likely star forming history. It is uncertain what fraction of massive stars actually do explode at the ends of their lives, as opposed to being, say, failed supernovae, but assuming it is all of them and using 1% for the high-mass fraction, the estimate of $3\times 10^{19}$ high-mass stars being born translates to a total number of stars in the (observable) universe of $\sim 3 \times 10^{21}$.

> **Comments:**
> **username_0**: >"1% of stars that are ever born will have M>8M⊙ and are candidates to end their lives as supernovae" ==[ I didn't know it was so low! But are no large blues being born at the top of the HR anymore? What about Rigel and Sirius? They're pop 1, right?  
> **username_2**: @Miss_Understands yes, massive stars are being born now. Just not as frequently as 10 Gyr ago.  Rigel and Sirius (Sirius is a relatively low-mass star and will not go supernova) are Pop 1.  
> **username_0**: what's the ratio of super blue star creation now compared to the first GY?  
> **username_2**: @Miss_Understands There is a plot referenced in https://astronomy.stackexchange.com/questions/46769/can-we-estimate-the-number-of-stars-which-have-died-in-the-observable-universe  
> **username_0**: thanx, perfesser! It looks like two orders of magnitude, but that's the strangest vertical axis I've ever seen, even in astronomy. Log of solar masses per year divided by the cube root of the distance!  
> **username_3**: @Miss_Understands it's a cube, not a cube root. Mass per year per volume. Mpc is just any old unit of length in this case, not distance from Earth.  
> **username_0**: >Mass per year per volume.  ==[ Yeah, that would be it. And whaddya know; it's simple! Thanx!  
> **username_4**: @username_2 *"Just not as frequently as 10 Gyr ago"* Does this mean that it would be more correct to say *"**almost** all the massive stars ever born have died"*?  

---

### Answer by username_1 on 2024-06-26. Score: 5

The other answer is right but it's interesting to see how the half a billion supernovae claim compares to what NASA has found.

According to the link, "[the JADES Transient Survey Sample team uncovered about 80 supernovae in a patch of sky only about the thickness of a grain of rice held at arm’s length](https://science.nasa.gov/missions/webb/nasas-webb-opens-new-window-on-supernova-science/#h-image-a-jades-deep-field-annotated)". That's not a rigorous measure of the field of view but assuming a 1mm x 1mm field at 1m distance, it covers 1e-6 stereo-radians. The whole sky is (4·pi)=12.56 stereo-radians. Therefore if that patch is a representative sample of the sky and the same survey could be done for the whole sky, we would expect to get 80·(4·pi)/(.001·.001)=1,005,309,649 supernovae, which is a more than half a billion but in the same order of magnitude. Taking a larger grain of rice or a shorter arm yields different numbers of the same order. A 1.5mm grain of rice and an 1m arm would lower the estimate to 446,804,289 supernovae for the whole sky.

Therefore, we can accept the half a billion supernovae as a reasonable ballpark estimate.

And just for completeness, 80 observations in a sample yields a 95% confidence interval (63.43502, 99.56692) for rice grain sized patches of sky using poisson.test in R. However the main uncertainty in this question is the size of the field of view.

> **Comments:**
> **username_2**: But interestingly, agrees with my 0.3 billion figure. Which is anpther way of saying that the observed SN rate is consistent with the known star forming rate of the universe and a universal IMF. However, my number does NOT account for observational incompleteness, which there surely must be. But I think JADES is sensitive enough to get the peak in the SN rate at $1<z<3$.  
> **username_1**: @PM2Ring - Thank you. I was misremembering the formula for volume instead of area. That's the reason I had to use such a tiny grain of rice to meet the claim.  

---

