How Quickly Does a Supernova Happen?

Article written: 9 Apr , 2015
Updated: 27 Feb , 2017
by

When a massive star reaches the end of its life, it can explode as a supernova. How quickly does this process happen?

Our Sun will die a slow sad death, billions of years from now when it runs out of magic sunjuice. Sure, it’ll be a dramatic red giant for a bit, but then it’ll settle down as a white dwarf. Build a picket fence, relax on the porch with some refreshing sunjuice lemonade. Gently drifting into its twilight years, and slowly cooling down until it becomes the background temperature of the Universe.

If our Sun had less mass, it would suffer an even slower fate. So then, unsurprisingly, if it had more mass it would die more quickly. In fact, stars with several times the mass of our Sun will die as a supernova, exploding in an instant. Often we talk about things that take billions of years to happen on the Guide to Space. So what about a supernova? Any guesses on how fast that happens?

There are actually several different kinds of supernovae out there, and they have different mechanisms and different durations. But I’m going to focus on a core collapse supernova, the “regular unleaded” of supernovae. Stars between 8 and about 50 times the mass of the Sun exhaust the hydrogen fuel in their cores quickly, in few short million years.

Just like our Sun, they convert hydrogen into helium through fusion, releasing a tremendous amounts of energy which pushes against the star’s gravity trying to collapse in on itself. Once the massive star runs out of hydrogen in its core, it switches to helium, then carbon, then neon, all the way up the periodic table of elements until it reaches iron. The problem is that iron doesn’t produce energy through the fusion process, so there’s nothing holding back the mass of the star from collapsing inward.
… and boom, supernova.

The outer edges of the core collapse inward at 70,000 meters per second, about 23% the speed of light. In just a quarter of a second, infalling material bounces off the iron core of the star, creating a shockwave of matter propagating outward. This shockwave can take a couple of hours to reach the surface.

Type II Supernovae

SN 1987A, an example of a Type II-P Supernova

As the wave passes through, it creates exotic new elements the original star could never form in its core. And this is where we get all get rich. All gold, silver, platinum, uranium and anything higher than iron on the periodic table of elements are created here. A supernova will then take a few months to reach its brightest point, potentially putting out as much energy as the rest of its galaxy combined.

Supernova 1987A, named to commemorate the induction of the first woman into the Rock and Roll Hall of Fame, the amazing Aretha Franklin. Well, actually, that’s not true, it was the first supernova we saw in 1987. But we should really name supernovae after things like that. Still, 1987A went off relatively nearby, and took 85 days to reach its peak brightness. Slowly declining over the next 2 years. Powerful telescopes like the Hubble Space Telescope can still see the shockwave expanding in space, decades later.

Evolution of a Type Ia supernova. Credit: NASA/CXC/M. Weiss

Evolution of a Type Ia supernova. Credit: NASA/CXC/M. Weiss

Our “regular flavor” core collapse supernova is just one type of exploding star. The type 1a supernovae are created when a white dwarf star sucks material off a binary partner like a gigantic parasitic twin, until it reaches 1.4 times the mass of the Sun, and then it explodes. In just a few days, these supernovae peak and fade much more rapidly than our core collapse friends.

So, how long does a supernova take to explode? A few million years for the star to die, less than a quarter of a second for its core to collapse, a few hours for the shockwave to reach the surface of the star, a few months to brighten, and then just few years to fade away.

Which star would you like to explode? Tell us in the comments below.

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10 Responses

  1. Steven says

    I would think the point where the iron fuses and takes energy away has to be communicated out through the layers. It could be dramatic but it will take some time to reach that (really?) 70,000 meters/sec. Or does the iron core itself just do the collapsing with the weight of the star above it?

    • Qev says

      Going by wikipedia, it takes several days for the iron core to accumulate from silicon burning. Once the core passes over the Chandrasekhar limit it collapses in milliseconds due to its own gravity.

    • bdonohue says

      No too long. It all depends on the gravity at these outer edges and how you define what an outer edge is. On our sun it’d take about 255 seconds to reach that velocity (presuming a constant gravitational gradient, which it isn’t, but good enough for this cheap example)
      (since v=gt thus t=v/g
      255 seconds = 70,000m/s /(27.901*9.8065m/s/s)
      where 27.901 is the “edge” gravity of the sun and 9.8065m/s/s is one gravity)) .

      If you had a “edge” gravity of 50g it’d take only 140 seconds.

  2. BlackWolfStanding says

    I would like to see the star Betelgeuse explode. I know it’s not due to explode for a few more years, but the science learned and verified will be tremendous.

    • FarAwayLongAgo says

      Be careful what you wish for, it might be a dangerous event. Astronomers don’t know where Betelgeuse is, maybe between 500 and 800 light years from here they guestimate (its variability makes it especially difficult to measure the distance to it). When it goes supernova it might leave behind a nasty neutron star or black hole with an accretion disc that sends a jet of high energy photons and atomic nuclei at relativistic speeds towards us, destroying our atmosphere and evaporating all water on Earth, killing all life.

      I hope I don’t cause any doomsday fear, it is a scenario which won’t happen in our lifetime. Even if such a thing forms, which isn’t certain, it will most likely not point towards us, nor be powerful enough to do any real damage. But since it will happen within a few thousands of years, and we the life on Earth has been around a million times longer, it maybe should make us feel lucky having come this far.

  3. l3cc says

    If the core collapse inwards at about 23% the speed of light then this would be about 70,000 kilometers (roughly 43,500 miles) per second rather than 70,000 meters. Is this correct or am I seeing it wrongly?

    • miguez says

      That’s why I just logged in to reply. 70,000 meters per second equates to 0.023% of the speed of light.

      • bdonohue says

        Yup. Unit conversion issue. m/sec vs km/sec.
        Light is 299,792,458 m/sec.
        70,000/299,792,458 m/sec = 0.00023 or 0.023%

  4. TedH says

    Let’s keep Beteigeuze in Orion and take one star of the Pleiades cluster… I kinda like Orion and it wouldn’t be “complete” without BigB. 🙂

  5. Member
    Cyrus says

    Good video – been waiting for this one, good commentary also. I select Berengeria Prime to supernova before our very eyes.

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