Galaxies and the Milky Way

A galaxy is a vast collection of stars held together by gravity. A galaxy contains many billions of stars, with gravity keeping the collection together.

The Milky Way is the galaxy that contains the Sun. The Sun is therefore one star among the many stars in the Milky Way.

The Sun is much closer to Earth than the other stars in the Milky Way. Although those other stars are in the same galaxy as the Sun, they are much farther away from Earth.

Light-year

A light-year is a unit of distance used to express the very large distances between objects in space. One light-year is the distance light travels through a vacuum in one year.

Although its name includes “year”, a light-year measures distance, not time. For example, if an object is four light-years away, its light travels for four years through a vacuum to reach us.

Converting light-years and metres

A light-year is a unit of distance, not time. One light-year is approximately $$9.5 \times 10^{15},\mathrm{m}$$. Metres and light-years therefore measure the same kind of quantity, but a light-year is much larger.

From light-years to metres

To convert a distance in light-years to metres, multiply by $$9.5 \times 10^{15}$$:

For example, $$2.0$$ light-years is approximately $$2.0 \times 9.5 \times 10^{15} = 1.9 \times 10^{16},\mathrm{m}$$.

From metres to light-years

To convert a distance in metres to light-years, divide by $$9.5 \times 10^{15}$$:

For example, $$1.9 \times 10^{16},\mathrm{m}$$ is approximately $$(1.9 \times 10^{16}) \div (9.5 \times 10^{15}) = 2.0$$ light-years.

Star formation and stability

A star begins in an interstellar cloud of gas and dust containing hydrogen. Gravity pulls the material in the cloud together, causing the cloud to collapse. As the material becomes concentrated, it forms a protostar. The protostar heats up as gravity continues to make it contract.

When the centre becomes hot enough, nuclear reactions begin: hydrogen acts as fuel and is converted into helium, releasing energy. The high temperature creates pressure that pushes outward against gravity, which continues to pull the star inward.

The star becomes stable when the outward force balances the inward gravitational force. Its size then remains roughly constant rather than continuing to contract. The nuclear reactions release energy that helps maintain the high temperature and pressure needed for this balance.

Evolution of less massive stars

A less massive star produces energy by nuclear fusion in its centre, where hydrogen is converted into helium. Eventually, most of the hydrogen in the centre is used up, so the star can no longer continue this process there as before.

The centre contracts and heats up, while the star’s outer layers expand. The expanded star has a cooler surface and becomes a red giant.

Later, the red giant sheds its outer layers. These layers form a cloud of gas called a planetary nebula. The hot, dense core left at the centre is a white dwarf. The white dwarf no longer produces energy through nuclear fusion and gradually cools.

Evolution of massive stars

A massive star produces energy by fusing hydrogen into helium in its centre. When most of the hydrogen there has been converted to helium, the core contracts and heats up, while the outer layers expand. The star becomes a red supergiant.

In the red supergiant, further fusion produces heavier elements. Eventually, the core collapses and the star explodes in a supernova. The explosion ejects material into space, forming a nebula that contains hydrogen and newly formed heavier elements.

The collapsed core remains at the centre as either a neutron star or a black hole, depending on the mass of the core. Material from the supernova nebula may later become part of new stars and their orbiting planets.