Supernova

Supernova

A blindingly bright star bursts into view in a corner of the night sky — it wasn't there just a few hours ago, but now it burns like a beacon.

That bright star isn't actually a star, at least not anymore. The brilliant point of light is the explosion of a star that has reached the end of its life, otherwise known as a supernova.

Supernovas can briefly outshine entire galaxies and radiate more energy than our sun will in its entire lifetime. They're also the primary source of heavy elements in the universe.

On average, a supernova will occur about once every 50 years in a galaxy the size of the Milky Way. Put another way, a star explodes every second or so somewhere in the universe.

Exactly how a star dies depends in part on its mass. Our sun, for example, doesn't have enough mass to explode as a supernova (though the news for Earth still isn't good, because once the sun runs out of its nuclear fuel, perhaps in a couple billion years, it will swell into a red giant that will likely vaporize our world, before gradually cooling into a white dwarf).

A star can go supernova in one of two ways:

• Type I supernova: star accumulates matter from a nearby neighbor until a runaway nuclear reaction ignites.
• Type II supernova: star runs out of nuclear fuel and collapses under its own gravity.

Let's look at the more exciting Type II first:

For a star to explode as a Type II supernova, it must be at several times more massive than the sun (estimates run from eight to 15 solar masses). Like the sun, it will eventually run out of hydrogen and then helium fuel at its core. However, it will have enough mass and pressure to fuse carbon. Here's what happens next:

• Gradually heavier elements build up at the center, and it becomes layered like an onion, with elements becoming lighter towards the outside of the star.
• Once the star's core surpasses a certain mass (the Chandrasekhar limit), the star begins to implode (for this reason, these supernovas are also known as core-collapse supernovas).
• The core heats up and becomes denser.
• Eventually the implosion bounces back off the core, expelling the stellar material into space ? the supernova.

What's left is an ultradense object called a neutron star.

There are sub-categories of Type II supernovas, classified based on their light curves. The light of Type II-L supernovas declines steadily after the explosion, while Type II-P's light stays steady for a time before diminishing. Both types have the signature of hydrogen in their spectra.

Stars much more massive than the sun (around 20 to 30 solar masses) might not explode as a supernova, astronomers think. Instead they collapse to form black holes.

Type I

Type 1 supernovas lack a hydrogen signature in their light spectra.

Type Ia supernovae are generally thought to originate from white dwarf stars in a close binary system. As the gas of the companion star accumulates onto the white dwarf, the white dwarf is progressively compressed, and eventually sets off a runaway nuclear reaction inside that eventually leads to a cataclysmic supernova outburst.

Astronomers use Type 1a supernovas as "standard candles" to measure cosmic distances because all are thought to blaze with equal brightness at their peaks.

Type 1b and 1c supernovas also undergo core-collapse just as Type II supernovas do, but they have lost most of their outer hydrogen envelopes.

Recent studies have found that supernovas vibrate like giant speakers and emit an audible hum before exploding.

In 2008, scientists caught a supernova in the act of exploding for the first time.

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Why Is It Dark At Night?

If Star Layer A is twice as far from Earth as Star Layer B, then the amount of light that reaches us from each star in A is only one-fourth the amount of light that reaches us from each star in B; but there are four times as many stars in A as there are in B.

The answer to this seemingly The answer to this seemingly simple question may boggle your brain. It's actually a famous cosmological problem, formally known as Olbers' Paradox. (Heinrich Olbers was a German astronomer who popularized discussion of this subject in 1826.) You might think that the question can be explained away by the effect of distance -- not so. To fully understand the perplexity, picture stars of equal brightness distributed evenly in concentric layers around Earth, like shells around a nut. The same amount oflight should reach Earth from each layer, because although the amount of light to reach us from each star decreases with distance (by 1/d^2), the number of stars in each layer increases, effectively balancing out the distance effect.

If the distance between A and B is 2 units, then each square in A is one-fourth as bright as each square in B; but there are four times as many squares in A as there are in B.

So light lost to distance does not account for the darkness of night. Obscuration by dust is not the answer, either, as any dust in the path of light would heat up and eventually reradiate. Most modern cosmologists have settled on two theories to account for the darkness. The first one states thatred shift (see Echo and Doppler Shift), which indicates that space itself is expanding, decreases the amount of light reaching us. The other explanation -- generally considered the main one -- is that the universe is not infinitely old. If it were, the sky would in fact be infinitely bright, because light from every point in the universe would have had time (eternity) to travel to every other point. As far as we know, there is no edge of the universe, only an edge of time. The finite age of the universe limits how much light we see.

One hundred billion galaxies, all full of millions of stars, seems like a lot but it isn’t nearly enough to make the night sky as bright as day. If there were an infinite number of stars and the universe was infinitely old, there would be a star everywhere you looked in the night sky and it would be very bright indeed.

The universe isn’t infinitely old. It was created approximately 14 billion years ago and since the speed of light is constant, we can only see objects that are less than 14 billion light years away. This means that we are living within a spherical ‘observable universe’ which is smaller that the total universe and that the light from stars further away from us than 14 billion light years will not have had enough time to reach the Earth.

In addition, the universe is expanding and all the galaxies, and their stars, are moving away from us. Thanks to this, the light from a moving star changes colour in a similar way that sound from a moving ambulance siren changes pitch. The light that we observe from distant receding stars is more red than it would be if they were stationary – the light is ‘red shifted’. In many cases the red shift is large enough to move the light out of the visible region of the electromagnetic spectrum.

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