Stars
Section: Space Physics | Syllabus: Cambridge AS Level Physics 9702
The Sun as a Star The Sun is a star of medium size - one of approximately 200 billion stars in our galaxy. It is the closest star to Earth, located about 150 million km away. The Sun consists mostly of hydrogen (~75%) and helium (~24%) It radiates most of its energy in the infrared (~50%), visible light (~40%) and ultraviolet (~10%) regions of the electromagnetic spectrum The Sun has a surface temperature of about 5800 K and a core temperature of about 15 000 000 K The Sun's mass (2 × 10 30 kg) is called the solar mass and is used to compare the mass of other stars The Sun contains over 99.8% of the total mass of the Solar System, giving it the strongest gravitational field of any object in the Solar System Nuclear Fusion (in Stars) Stars are powered by nuclear reactions that release energy.
In stable stars like the Sun, these reactions involve the fusion of hydrogen into helium. Hydrogen nuclei combine at extremely high temperatures and pressures to form helium, releasing large amounts of energy.
This is what makes stars shine. Stability of Stars In a stable star, the inward gravitational force (which would cause the star to collapse) is balanced by an outward force from the high-temperature core (radiation pressure from nuclear fusion).
When these forces are balanced, the star maintains a constant size. Figure: Force Balance in a Stable Star A circle representing the cross-section of a stable star. The star's core is labelled at the centre as "Core - nuclear fusion of H → He, very high temperature".
Multiple red arrows point outward from the core towards the surface, labelled "Outward radiation pressure (from high temperature)". Multiple blue arrows point inward from the surface towards the core, labelled "Inward gravitational force (pulls star together)".
The two sets of arrows are equal in length, indicating force balance. A label at the bottom reads "Forces balanced → stable star, constant size". A second diagram (smaller, beside the first) shows what happens when hydrogen runs out: the outward arrows become shorter than the inward arrows, with a label "Fusion slows → gravity wins → star contracts".
Galaxies Galaxy A massive system of many billions of stars held together by gravity. There are an estimated 200 billion galaxies in the observable Universe. The Sun is a star in a galaxy called the Milky Way The Milky Way contains approximately 200 billion (2 × 10 11 ) stars It is a spiral galaxy with a central bulge, with a diameter of about 100 000 light-years Our Solar System is located about 30 000 light-years from the galactic centre , two-thirds of the way along a spiral arm Other stars in the Milky Way are much further from Earth than the Sun is - the Sun is the closest star to us The nearest star after the Sun is Proxima Centauri , about 4.2 light-years away Figure: The Milky Way Galaxy Two views of the Milky Way.
Top-down view: a spiral galaxy with a bright central bulge and several spiral arms. The position of our Solar System is marked approximately two-thirds out from the centre along one of the spiral arms, about 30 000 light-years from the galactic centre.
The total diameter of 100 000 light-years is labelled. Side-on view: a lens-shaped disc with a central bulge, approximately 2000 light-years thick. Light-Years The distances between stars and galaxies are so enormous that ordinary units (km) are impractical.
Astronomers use the light-year as a unit of distance. Light-Year The distance that light travels through a vacuum in one year . It is a unit of distance , not time. 1 light-year = 9.5 × 10 15 m Calculating One Light-Year distance = speed × time = (3 × 10 8 m/s) × (365.25 × 24 × 3600 s) = (3 × 10 8 ) × (3.156 × 10 7 ) = 9.5 × 10 15 m Object / Scale Distance from Earth Light Travel Time The Sun 1.5 × 10 11 m (150 million km) ~8 minutes Proxima Centauri (nearest star) 4.2 light-years 4.2 years Across the Milky Way ~100 000 light-years 100 000 years Key Idea When you observe a star 4.2 light-years away, you are seeing light that left it 4.2 years ago .
Looking at distant objects in space means looking back in time. The most distant galaxies we can observe show us what the Universe looked like billions of years ago.
Interactive revision notes, videos and practice questions load below.