# What observation can be expected on LIGO if any when Betelgeuse goes supernova?

What observation can be expected from the LIGO gravitational wave observatory if any when Betelgeuse goes supernova?

Could we know that Betelgeuse has gone supernova before we see it light up our night sky?

or other current or soon-to-be completed gravitational wave obervatories

• I made some small edits to your (really interesting) question, feel free to roll back or edit further, thanks! – uhoh Dec 26 '19 at 1:47
• The question mentions specific gravitational wave observatories. Do you require answers to take into account what the observatories are (or will be) capable of measuring, or are you looking for what could theoretically be measured, if the instruments were capable of measuring it? For instance, LIGO and VIRGO are tuned for stellar mass neutron star/black hole merger events, and may be unlikely to detect any gravitational waves from a supernova. – asgallant Dec 26 '19 at 18:14
• @asgallant The papers I have looked at suggest that a SN at the distance of Betelgeuse would be easily detected by LIGO. – Rob Jeffries Dec 26 '19 at 21:30
• The missing context here seems to be that there is actually a suggestion that Betelgeuse could go supernova soon: washingtonpost.com/science/2019/12/27/… – Ben Crowell Dec 27 '19 at 23:10

Potentially a short (less than a second) burst of gravitational waves (GWs) would be detected. Much depends on asymmetries in the core collapse, since a spherically (or even axially) symmetric collapse would not produce GWs (e.g. Morozova et al. 2019). However, theoretical models suggest that the GWs start at low frequency (tens of Hz) and are associated with the core bounce at the end of core collapse. After about half a second the frequencies rise to kHz and are associated with the oscillations of the proto-neutron star at the centre. Since LIGO is sensitive from about 10Hz to 1kHz, then only the initial phase that lasts a few tenths of a second would be detectable, but snce GWs can travel through the star without much hindrance they will indicate the moment of core collapse quite accurately.

The theoretical calculations referenced above suggest GW amplitudes of $$h_{+}d \sim 10$$, where $$h_{+}$$ is the dimensionless GW strain and $$d$$ is the distance to the supernova in cm. Betelgeuse is at a distance of $$\sim 6 \times 10^{20}$$ cm (600 light years), so the strain at the detector would be $$h_{+} \sim 2\times 10^{-20}$$. LIGO is sensitive to strains of above $$10^{-23}$$ in this frequency range, so there should be no problem (according to these models) in detecting such an event.

If they were produced with detectable strength, then they should be received at the same time as ananticipated neutrino burst. The difference would be that the neutrino burst should extend over about 10 s. That is because the dense centre of a supernova is opaque even to neutrinos (but not gravitational waves) for that period and so they "leak out" over that period.

The electromagnetic signature of the supernova (i.e. a visible light increase) would be seen a few hours after that, since that is how long it takes for energy to be transferred to the envelope of the star.

NB: For all practical intents and purposes, over the distance to Betelgeuse, neutrinos, along with light and gravitational waves, travel at the same speed. i.e. The delay of the ($$\sim 10$$ MeV) neutrino burst due to their non-zero ($$\ < 1$$ eV) mass would be negligible. If the Lorentz factor is $$10^7$$, then the neutrino speed is the same as the speed of light to 1 part in $$2\times 10^{14}$$, which means they take an extra 0.1 ms to arrive from Betelgeuse compared to light speed.

• Newbie question: how is a spherical ly symmetrical collapse possible, it is rotating i assume? Can core collapse happen without any effect of the torque perturbing that symmetry? – Stian Yttervik Dec 26 '19 at 16:16
• @StianYttervik That would be axially symmetric. Spherically symmetric collapse is unlikely - as you suggest. – Rob Jeffries Dec 26 '19 at 17:03

• Betelgeuse is currently undergoing a "low-state" period, which is the combined effect of two periodic variations (See for instance http://www.astronomerstelegram.org/?read=13365). Hence, there is no clear evidence right now that it will blow up as a core-collapse supernova, say, wihtin the next months.

• We are able to know if a core-collapse supernova occurs before optical light reaches us thanks to the detection of neutrinos. This was the case for the famous SN1987A in the Magellanic Clouds (https://en.wikipedia.org/wiki/SN_1987A#Neutrino_emissions, 2-3 hours before optical light)

• LIGO is already operational (https://en.wikipedia.org/wiki/LIGO)

• As far as I can tell, the merging of two neutron stars or two black holes releases more energy and produces a stronger gravitational wave that a "simple" collapse of a massive star (but this is a non-documented opinion). But Betelegeuse being a lot closer than the current detected sources of Gravitational Waves, this might be an interesting target indeed (although, see first point...)

• Since Betelgeuse is about 700 light years away, it conceivably could have gone supernova well before LIGO became operational (or was even imagined), say 350 years ago? And we won't know until 700 years after it "actually" happened. (Ignoring relativistic concerns about simultaneity. I'm positing a useful reference frame that includes both the Sun and Betelgeuse close to relative rest.) – Ross Presser Dec 26 '19 at 16:05
• It would be a little better to be clear that we don't have a telescopic observation of first light from SN1987A to compare with the neutrino observations, so the delay you quote is very much an informed guess. – dmckee --- ex-moderator kitten Dec 26 '19 at 20:47
• I'd be a little bit cautious about saying that the current dimming is due to the confluence of "two periodic variations", largely because of the uncertainty in the long-term behavior - although yes, I agree with downplaying the pre-supernova hypothesis. – HDE 226868 Dec 26 '19 at 21:35
• @RossPresser If Betelgeuse is exactly 700 light years away and went supernova exactly 350 years ago, we will detect the gravitational waves from it in exactly 350 years because gravitational waves travel at the speed of light. – CJ Dennis Dec 27 '19 at 6:20
• @RossPresser I think it's safe to say that all events detected by LIGO happened before it was built since building commenced 25 years ago and there are no objects we know of within 25 light years that could cause a detectable event. – CJ Dennis Dec 27 '19 at 22:03