In the music world, the death of a star is precipitated by sex, drugs and rock-n-roll. In the Universe, the death of a star is precipitated by extinguishing the nuclear fusion reactions in its stellar furnace. The end result is often one of the most fascinating objects in the universe, a pulsating neutron star, “pulsar” for short.
Neutron stars hold the key to what Dr. Elizabeth Plimpton had written on her hand in this episode.
The life of a star is a constant tug-of-war. The force of gravity never ceases pulling all the star’s material inward, attempting to make it smaller and denser. But a denser star would accelerate the nuclear fusion reactions, raising the star’s temperature. Like heating a pan of jiffy-pop, the heat causes an outward pressure, trying to making the star larger. For much of a star’s life the forces strike a balance and stars such as our own stay pretty much the same size for billions of years.
But gravity always wins. Fo now,our own Sun shines largely by turning hydrogen into helium. By making a more tightly bound nucleus in this reaction, energy is left over and produces the light and heat of the Sun. About 5 billion years from now, after its hydrogen is used up, our Sun will turn to alternative energy, fusing helium into carbon and oxygen, becoming much larger and growing briefly into a Red Giant star (while engulfing Mercury and Venus, and incinerating the Earth) in the process. But that’s all folks. Our Sun will finally run out of energy, and puff off all the excess material. The Sun’s now naked core will quietly cool. Gravity will pull it tighter and tighter until the electrons in the Sun resist being pushed any closer together. The remaining ember of carbon and oxygen is incredibly dense. A white dwarf with the mass of the Sun will be the size of only the Earth. It really isn’t even much of star any more since it is no longer producing its own energy. It only glows by radiating the energy from its former life, a cosmic Zsa Zsa Gabor.
But a star more massive than our Sun face a different fate. A larger star does not go quietly but often blows off material in a dramatic supernova explosion followed by a gravitational collapse of the remaining core. The inward pressure due to gravity is so great that the electrons that hold up a White Dwarf are “pushed into” the remaining protons to form neutrons. The stellar material now moves even further inward under the force of gravity to make an object as dense as an atomic nucleus. Since it is mostly made of neutrons, it is called a neutron star. A neutron star 3 times the mass of Sun is so small and dense, it is smaller than Los Angeles.
It is so dense, that one teaspoonful of neutron star material here on Earth would weigh as much as a mountain. As Dr. Plimpton says in the episode, if you went even close to its surface you’d be crushed by its strong gravity. That is if not first ripped limb-from-limb first by the differences in its strong force of gravity on different sides of your body.
But a neutron star’s useful life is far from over. While a graduate student Jocelyn Bell and her thesis advisor Anthony Hewish discovered regular bursts, from seconds to fractions of a second, of radio static from specific points in the galaxy. These turned out to be the fast spinning remnant neutron stars. Just as when a slowly rotating Olympic skater pulls his or her arms inward to speed up, the small neutron star remnant of a star that probably rotated about once per few weeks, now rotates every few seconds or even faster. When the poles of the neutron star point at us on Earth we see a burst of radio and other light. Just as the spinning lamp in a lighthouse produces a flash of light to those at sea, we on Earth see a bursts of energy from the pulsar as it rotates. For this and her career’s work, Dr. Bell-Burnell was awarded the highest rank an British citizen can attain, Dame of the Britsh Empire.
And just as the spinning lamps in a lighthouse produce a regular flash of light for ships, cosmic voyagers would see these pulsars, spinning neutron stars, as regular and bright beacons from afar. The Pioneer spacecrafts launched in the 1970s are now leaving the solar system. With them they take our calling card on a gold-plated plaque. To instruct whoever or whatever discovers them how to find us, we show them the Earth relative to pulsars, cosmic beacons that will be visible throughout our portion of the Milky Way.
I’m told Carl Sagan caught hell for putting naked pictures in space. So for the subsequent Voyager spacecrafts, now the farthest spaceprobe from Earth, we instead sent “the golden record”…
…which sends our regards with more puritanical messages. The record is our ultimate mix tape to our alien friends. If we humans were to launch such a space probe now, I fear it would only have a golden MP3. Voyager 1 has left the Solar System and is about 110 times further than the Earth is from the Sun and is our most distant space probe. It will leave our solar system around 2015 and carry out message into interstellar space.
But as Steven Hawking points out, this might not have been such a good idea. Just ask the ancient Aztecs how much they benefited from the visits of the Spanish explorers. Even if we are visited by a species that is not violent, visitors may inadvertently bring microbes we’ve never been exposed to before that wipe us out. This may even be an inevitable by-product of all such contacts. If you haven’t read it, I highly recommend the masterpiece Guns Germs & Steel written by Jared Diamond (also at UCLA) who gives a scientific basis to the unfolding of such historical events. But the sword cuts both ways… In H.G. Wells’s War of the Worlds, it is our own microbes that ultimately killed the Martian visitors.
So it was the location of a new pulsar, a new neutron star, that Elizabeth Plimpton had written on her hand for tonight’s episode. A brand-new one (a “Soft Gamma-ray Repeater” at RA 4h40m, Dec 55035′, which is effectively its longitude and latitude on the sky) was announced on the The Astronomer’s Telegram before the episode was taped. In another easter-egg to the High-Def enabled, note the shout-out to Brian Greene’s excellent popular science book, in the title of Dr. Plimpton’s book: The Effervescent Universe.
No matter how much we hide, our radio and television transmissions are already giving us away, at the speed of light. Even tonight’s episode is already beyond Mars. The extra-terrestrials can easily find us, and may already be on their way.