Context

Astronomers in the U.K. said this week that, for the first time, they saw a white dwarf star abruptly switch on and off.

Background

• The star is TW Pictoris – 1,400 light-years from Earth – already known as a variable star. But its previous brightness changes were seen to happen over days or months.
• This time, astronomers saw the star suddenly dim and brighten again over the course of just 30 minutes.
• The TW Pictoris system includes a white dwarf, plus a small companion star that the white dwarf “feeds” on. The white dwarf pulls in material from the companion.
• It’s this gradual accretion (build up) of material that causes the dwarf to change its brightness.

Analysis

About Stars

• A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity.
• A star's life begins with the gravitational collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements.
• The total mass of a star is the main factor that determines its evolution and eventual fate.
• For most of its active life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and then radiates into outer space.
• At the end of a star's lifetime, its core becomes a stellar remnant: a white dwarf, a neutron star, or, if it is sufficiently massive, a black hole.
• Stars can form orbital systems with other astronomical objects, as in the case of planetary systems and star systems with two or more stars.
  

Different types of Stars

• Protostar:
  • A protostar is what you have before a star forms.
  • A protostar is a collection of gas that has collapsed down from a giant molecular cloud.
  • The protostar phase of stellar evolution lasts about 100,000 years.
  • Over time, gravity and pressure increase, forcing the protostar to collapse down.
  • All of the energy released by the protostar comes only from the heating caused by the gravitational energy – nuclear fusion reactions haven’t started yet.
• T Tauri Star:
  • A T Tauri star is a stage in a star’s formation and evolution right before it becomes a main sequence star.
  • This phase occurs at the end of the protostar phase, when the gravitational pressure holding the star together is the source of all its energy.
• Main Sequence Star:
  • The majority of all stars in our galaxy, and even the Universe, are main sequence stars. Our Sun is a main sequence star, and so are our nearest neighbors, Sirius and Alpha Centauri A.
  • Main sequence stars can vary in size, mass and brightness, but they’re all doing the same thing: converting hydrogen into helium in their cores, releasing a tremendous amount of energy.
  • A star in the main sequence is in a state of hydrostatic equilibrium. Gravity is pulling the star inward, and the light pressure from all the fusion reactions in the star are pushing outward.
  • The inward and outward forces balance one another out, and the star maintains a spherical shape.
  • Stars in the main sequence will have a size that depends on their mass, which defines the amount of gravity pulling them inward.
• Red Giant Star:
  • When a star has consumed its stock of hydrogen in its core, fusion stops and the star no longer generates an outward pressure to counteract the inward pressure pulling it together.
  • A shell of hydrogen around the core ignites continuing the life of the star, but causes it to increase in size dramatically.
  • The aging star has become a red giant star
  • When this hydrogen fuel is used up, further shells of helium and even heavier elements can be consumed in fusion reactions.
  • The red giant phase of a star’s life will only last a few hundred million years before it runs out of fuel completely and becomes a white dwarf.
• White Dwarf Star:
  • When a star has completely run out of hydrogen fuel in its core and it lacks the mass to force higher elements into fusion reaction, it becomes a white dwarf star.
  • The outward light pressure from the fusion reaction stops and the star collapses inward under its own gravity.
  • A white dwarf shines because it was a hot star once, but there’s no fusion reactions happening any more.
• Red Dwarf Star:
  • Red dwarf stars are the most common kind of stars in the Universe.
  • These are main sequence stars but they have such low mass that they’re much cooler than stars like our Sun.
  • Red dwarf stars are able to keep the hydrogen fuel mixing into their core, and so they can conserve their fuel for much longer than other stars.
  • Astronomers estimate that some red dwarf stars will burn for up to 10 trillion years.
• Neutron Stars:
  • If a star has between 1.35 and 2.1 times the mass of the Sun, it doesn’t form a white dwarf when it dies. Instead, the star dies in a catastrophic supernova explosion, and the remaining core becomes a neutron star.
  • As its name implies, a neutron star is an exotic type of star that is composed entirely of neutrons.
  • This is because the intense gravity of the neutron star crushes protons and electrons together to form neutrons.
  • If stars are even more massive, they will become black holes instead of neutron stars after the supernova goes off.
• Supergiant Stars:
  • The largest stars in the Universe are supergiant stars.
  • Unlike a relatively stable star like the Sun, supergiants are consuming hydrogen fuel at an enormous rate and will consume all the fuel in their cores within just a few million years.
  • Supergiant stars live fast and die young, detonating as supernovae; completely disintegrating themselves in the process.

Characteristics of Star

• Age
  • Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.8 billion years old—the observed age of the universe.
  • The oldest star yet discovered, HD 140283, nicknamed Methuselah star.
  • The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly.
• Chemical composition
  • When stars form in the present Milky Way galaxy, they are composed of about 71% hydrogen and 27% helium, as measured by mass, with a small fraction of heavier elements.
• Diameter
  • Stars range in size from neutron stars, which vary anywhere from 20 to 40 km (25 mi) in diameter, to supergiants like Betelgeuse in the Orion constellation, which has a diameter about 1,000 times that of the Sun with a much lower density.
• Kinematics
  • The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy.
  • The components of motion of a star consist of the radial velocity toward or away from the Sun, and the traverse angular movement, which is called its proper motion.
• Magnetic field
  • The magnetic field of a star is generated within regions of the interior where convective circulation occurs.
  • This movement of conductive plasma functions like a dynamo, wherein the movement of electrical charges induce magnetic fields, as does a mechanical dynamo.
  • Those magnetic fields have a great range that extend throughout and beyond the star.
  • The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation.
• Temperature
  • The surface temperature of a main sequence star is determined by the rate of energy production of its core and by its radius, and is often estimated from the star's color index.

Radiation by Stars

• The energy produced by stars, a product of nuclear fusion, radiates to space as both electromagnetic radiation and particle radiation.
• The particle radiation emitted by a star is manifested as the stellar wind, which streams from the outer layers as electrically charged protons and alpha and beta particles.
• A steady stream of almost massless neutrinos emanate directly from the star's core.
• The color of a star, as determined by the most intense frequency of the visible light, depends on the temperature of the star's outer layers, including its photosphere.
• Besides visible light, stars emit forms of electromagnetic radiation that are invisible to the human eye.
• In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum, from the longest wavelengths of radio waves through infrared, visible light, ultraviolet, to the shortest of X-rays, and gamma rays.

Conclusion

Most of what we can learn about the Universe is based on observing stars. Fortunately, stars are profoundly important for our Universe, even though they do not dominate its total material (most is in a dark form that we know little about). Not only do they light up the sky, they produce the raw materials that make life possible, and if there is life out there, it is most likely orbiting a star on its planet.