Stars Revision Notes

A galaxy is a massive system consisting of billions of stars, along with gas, dust, and dark matter, all bound together by gravity. The Milky Way is the galaxy that contains our Solar System. Galaxies vary in size and shape but share the common feature of containing a vast number of stars.

Stars within a galaxy are distributed in different regions such as the central bulge, spiral arms (in spiral galaxies like the Milky Way), and the halo. The immense number of stars in a galaxy means that individual stars are generally very far apart from each other compared to their sizes.

• Galaxies contain billions of stars.
• They also contain gas, dust, and dark matter.
• Gravity holds all components of a galaxy together.
• The Milky Way is the galaxy that includes our Solar System.

The Sun is a star located in one of the spiral arms of the Milky Way galaxy, approximately 27,000 light-years from the galactic centre. It is one of billions of stars in the Milky Way but is much closer to Earth than any other star.

Because the Sun is our nearest star, it appears much brighter and larger in the sky than other stars, which are much farther away. The distances to other stars in the Milky Way are so great that they cannot be measured in conventional units like kilometres or miles.

• The Sun is a star within the Milky Way galaxy.
• It is located in a spiral arm about 27,000 light-years from the centre.
• The Sun is much closer to Earth than other stars in the Milky Way.
• Other stars appear dimmer due to their much greater distances.

Distances between stars and galaxies are extremely large, so astronomers use special units to express these distances conveniently. The most common unit for measuring distances within and between galaxies is the light-year.

A light-year is defined as the distance that light travels in one year through a vacuum. Since light travels at a constant speed of approximately 3.0 × 10^8 metres per second, the distance covered in one year is enormous.

Using light-years allows astronomers to express vast distances in manageable numbers and also relates distance to the time taken by light to travel from distant objects to Earth.

• Astronomical distances are very large and require special units.
• A light-year is the distance light travels in one year in a vacuum.
• Light speed, c, is approximately 3.0 × 10^8 m/s.
• Light-year distance = speed of light × time (one year).

To calculate the length of one light-year, use the formula: distance = speed × time.

Speed of light, c = 3.0 × 10^8 m/s (in vacuum). Time in one year:

1 year = 365.25 days × 24 hours/day × 3600 seconds/hour = 31,557,600 seconds (accounting for leap years). Therefore: distance = 3.0 × 10^8 m/s × 31,557,600 s = 9.47 × 10^15 metres (approximately). This distance is about 9.5 trillion kilometres, which is far beyond everyday scales but suitable for expressing distances between stars and galaxies.

• Formula: distance = speed × time.
• Speed of light, c = 3.0 × 10^8 m/s.
• Time in one year ≈ 3.16 × 10^7 seconds.
• One light-year ≈ 9.5 × 10^15 metres.

A light-year is the distance that light travels in one year through a vacuum. It is a unit of length used primarily in astronomy to express extremely large distances between stars, galaxies, and other celestial objects.

Since light travels at a constant speed in a vacuum, the light-year provides a convenient way to measure how far light can travel over the course of a year. This helps astronomers describe distances that are otherwise too large to express in kilometres or metres easily.

• Light-year is a unit of distance, not time.
• It is based on the speed of light in a vacuum.
• Used to measure distances between stars and galaxies.

The speed of light in a vacuum, denoted by c, is approximately 3.0 × 10⁸ metres per second (m/s). This is a fundamental constant of nature.

To calculate the distance light travels in one year, multiply the speed of light by the number of seconds in one year.

Number of seconds in one year = 365 days × 24 hours/day × 60 minutes/hour × 60 seconds/minute = 31,536,000 seconds (approximately).

• Speed of light, c = 3.0 × 10⁸ m/s
• Time in one year, t = 3.1536 × 10⁷ s
• Distance = speed × time

Using the formula: distance = speed × time, substitute the known values:

distance = (3.0 × 10⁸ m/s) × (3.1536 × 10⁷ s) = 9.4608 × 10¹⁵ metres.

This is commonly rounded to 9.5 × 10¹⁵ metres for simplicity in calculations and communication.

• Formula: d = c × t
• Substitution: d = 3.0 × 10⁸ × 3.1536 × 10⁷
• Answer: d ≈ 9.5 × 10¹⁵ m

Distances in space are extremely large, making conventional units like metres or kilometres impractical for expressing distances between stars or galaxies.

The light-year provides a scale that is easier to understand and compare. For example, the nearest star to Earth (other than the Sun) is about 4.3 light-years away, meaning light takes 4.3 years to travel from that star to Earth.

Using light-years also helps relate distance to the time taken by light to travel, linking spatial and temporal concepts in astronomy.

• Astronomical distances are vast and require large units.
• Light-year relates distance to time taken by light.
• Facilitates understanding of space scales.

Stars begin their life cycle in vast clouds of interstellar gas and dust, primarily composed of hydrogen. These clouds are called nebulae. Gravity causes regions within these clouds to collapse, increasing the density and temperature of the material.

As the cloud collapses, gravitational potential energy converts into thermal energy, heating the core. This process continues until the core temperature is sufficient for nuclear fusion to begin.

The initial collapsing object is called a protostar. It is not yet a true star because nuclear fusion has not started in its core.

• Interstellar clouds mainly contain hydrogen gas and dust particles.
• Gravitational collapse increases density and temperature.
• Protostar forms as the cloud contracts but before fusion starts.

When the core temperature of the protostar reaches about 10 million kelvin, hydrogen nuclei begin to fuse into helium nuclei, releasing energy.

This energy produces thermal pressure that pushes outward, balancing the inward pull of gravity. This balance is called hydrostatic equilibrium and stabilises the star.

At this stage, the star enters the main sequence phase, where it remains stable for most of its life, steadily fusing hydrogen into helium in its core.

• Nuclear fusion starts at core temperatures ~10⁷ K.
• Thermal pressure from fusion balances gravitational force (hydrostatic equilibrium).
• Star is stable and on the main sequence during hydrogen fusion.

Over millions to billions of years, the star uses up the hydrogen fuel in its core, converting it into helium.

Once the hydrogen is depleted, fusion in the core slows or stops, causing the core to contract under gravity.

The outer layers expand and cool, turning the star into a red giant (for less massive stars) or a red supergiant (for more massive stars). This expansion increases the star's radius and decreases its surface temperature, giving it a reddish colour.

• Core hydrogen fusion ends as fuel runs out.
• Core contracts, outer layers expand and cool.
• Star becomes a red giant or red supergiant depending on mass.

In stars with masses similar to or less than the Sun, the red giant phase ends when the outer layers are expelled into space, forming a glowing shell of gas called a planetary nebula.

The remaining core becomes a white dwarf: a very dense, hot, but small star that no longer undergoes fusion.

White dwarfs gradually cool and fade over billions of years.

• Less massive stars shed outer layers as planetary nebulae.
• Core remains as a white dwarf, supported by electron degeneracy pressure.
• White dwarfs cool slowly without fusion.

More massive stars become red supergiants and continue fusion of heavier elements in their cores until iron forms, which cannot release energy by fusion.

When fusion stops, the core collapses rapidly, causing a supernova explosion that ejects the outer layers into space.

The supernova remnant is a nebula enriched with heavy elements formed during the explosion, contributing to the cosmic abundance of elements heavier than helium, essential for forming new stars, planets, and life.

• Massive stars fuse elements up to iron in their cores.
• Core collapse triggers a supernova explosion.
• Supernova ejects heavy elements into space, forming nebulae.
• New stars and planets can form from supernova remnants.