Galaxies and the universe

The Universe is vast, containing billions of galaxies, including our own—the Milky Way, which has a diameter of approximately 100,000 light-years. Observations of light from distant galaxies provide evidence that the Universe is expanding, supporting the Big Bang Theory.

Galaxies and astronomical distances

(a) Galaxies:

• Galaxies are massive systems that contain billions of stars along with gas, dust, and dark matter, all held together by gravity. The Milky Way is our home galaxy, and it is just one of many billions of galaxies in the universe.

(b) The Sun and the Milky Way:

• The Sun is one of the stars that make up the galaxy known as the Milky Way. Our Solar System is located within one of the spiral arms of the Milky Way.

(c) Stars Beyond the Sun:

• The other stars that make up the Milky Way are much further away from the Earth than the Sun is. The Sun is the closest star to us, and other stars are separated by vast distances.

(d) Measuring Astronomical Distances:

• Astronomical distances are so vast that they are measured in light-years.

• A light-year is the distance that light travels in a vacuum in one year. Since light travels at about 300,000 km/s, the distance covered in a year is enormous.

The milky way

• The Milky Way is a spiral galaxy that contains our Solar System and hundreds of billions of stars.

• It is just one of billions of galaxies in the Universe.

• The diameter of the Milky Way is about 100,000 light-years, meaning that light would take 100,000 years to travel from one side to the other.

Red shift

Redshift is the increase in the wavelength of light from a star or galaxy, making it appear shifted toward the red end of the electromagnetic spectrum. It occurs when an object is moving away from the observer due to the Doppler effect.

Explanation of Redshift

• Similar to how a siren's pitch lowers as an ambulance moves away, light waves from distant galaxies stretch, increasing their wavelength and shifting toward the red part of the spectrum.

• The greater the redshift, the faster a galaxy is moving away.

Red shift and distant galaxies

• Light from distant galaxies appears more redshifted compared to light emitted on Earth.

• This tells us that distant galaxies are moving away from us.

• The further a galaxy is from Earth, the greater the redshift, meaning it is moving away faster.

Evidence for the expanding universe