::Interactive Astronomy::

Main Sequence Stars

Proto stars will evolve over a few million years to become main sequence stars. This can also be seen in the Hertzsprung-Russell Diagram.

The bright blue-white main sequence star Vega (26 light-years away) in the constellation of Lyra.

For stars the mass of our sun

The surface temperature of a contracting proto star about the mass of our sun would stay roughly constant although the luminosity decreases as the radius decreases. This is because the outer layers are cool and opaque, therefore energy released from the shrinking layers in radiation cannot reach the surface. It can only flow outward through convection, which is slower and less effective. The internal temperature increases over time and the interior becomes ionized. This allows the energy to escape more easily and the luminosity increases when energy is conveyed outward by radiation in the interior and convection in the outer layers.

For stars of mass more than our sun

More massive proto stars contract and heat more quickly and hydrogen burning begins earlier. Luminosity therefore stabilizes quickly while the surface temperature increases as the star shrinks. A massive star’s outer layers are of such little density that energy flows through them faster by radiation than by convection.

A diagram illustrating the different properties of proto stars

Universe, Kaufmann and Freedman 2002, Page 462

Ejection of mass

During the birth process, stars also still continue to eject their mass. A type of stars- T Tauri proto stars- often ejects matter into space in huge clouds. They do so along two narrow, oppositely directed jets- bipolar outflow.

This event causes Herbig-Haro objects to occur in the form of glowing knots as the material ejected at high speeds into space collides into interstellar medium. This outflow may be so much over a few thousand to million years in this stage of stellar evolution that the final product any even be of less mass than the original.

One reason cited for the bipolar outflow involves the magnetic field of the dark nebula from which the star forms. The magnetic field lines are dragged when material falls in. As the material in the disk is orbiting at varying speeds, this can distort the magnetic field lines into two helix shapes on opposite sides of the disc. This channels in falling material away from the proto star forming the jets.

Addition of mass

Besides ejecting matter, proto stars also add mass to themselves. The proto star’s nebula spins faster as it contracts and flattens. Particles orbiting the proto star will collide and spiral into the proto star, adding to its mass. This is known as accretion, the material is continually added to the circumstellar accretion disc.

Formation of Proto planetary Discs

Proto planetary discs or proplyds are discs that form around new protostars and are thought to be remnants of the circumstellar accretion disc after much of the material has been ejected by bipolar outflows. This may later coalesce to form planets around the star.

Evolution to become main-sequence stars- Core hydrogen burning

A freshly formed main-sequence star is called a zero-age main-sequence star. When the transition to a main-sequence star has taken place, changes in luminosity, surface temperature and radius will occur as a result of core hydrogen burning which alters the chemical composition of the core. Over time, there would be more helium than hydrogen atoms in the star’s core, which is also what is happening to our Sun now.

The pressure in the star’s core is increased as a result of there being now fewer particles due to core hydrogen burning which converts 4 hydrogen nuclei into 1 helium nucleus. Compression causes the core to be denser and increases its temperature. This, in turn causes the star to shine more luminously as the rate of hydrogen burning is increased.

A star’s main-sequence lifetime depends on the mass of the star. The more massive a star, the shorter its lifespan would be.