cosmological redshifts are calculated from observations of spectral lines from a. individual stars in distant galaxies. b. clouds of dust and gas in distant galaxies. c. spectra of entire galaxies. d. rotations of the disks of distant galaxies.

The universe is expanding. Imagine a balloon with tiny dots on it. As you blow up the balloon, the dots move away from each other. In a similar way, galaxies in our universe move away from each other as space itself expands. This causes light traveling through space to stretch out and move toward the red end of the spectrum. This phenomenon is called cosmological redshift.
Cosmological redshifts are essential in understanding how the universe evolves. By studying the redshift, we gain clues about the rate of expansion and the distance of far-off galaxies. Edwin Hubble first discovered this concept, leading to what we now call Hubble’s Law. This law tells us that the farther away a galaxy is, the faster it seems to be moving away from us.
This helps astronomers trace the history and future of our universe, showing that it's been expanding for billions of years and will likely continue to do so.

Spectral lines are fingerprints of elements and molecules. They're unique patterns of light absorbed or emitted by substances. When we look at the light from a star or galaxy, we can see these lines. They help us identify what the star or galaxy is made of.
But there's more! Because of the movement and expansion of the universe, these lines don't always appear where we expect them to be. If the object emitting the light is moving away, the lines shift to the red end of the spectrum – this is redshift. If it’s moving closer, the lines shift to the blue end – this is blueshift.
By carefully studying these lines, astronomers can determine not only the composition but also the velocity and distance of celestial objects. It’s like having a cosmic map that tells us the speed and direction of every galaxy we study.

When observing the universe, astronomers look at the spectra of entire galaxies. A spectrum is like a rainbow – it's the spread of light into its different colors. For galaxies, the spectrum includes light from billions of stars and clouds of gas and dust.
By studying the galactic spectra, we get a comprehensive view of the galaxy's overall properties. This is crucial for measuring cosmological redshifts because it provides an average redshift for the entire galaxy. Since individual stars are often too faint, and dust and gas clouds can be influenced by local motion, entire galaxies provide the most reliable redshift data.
Galactic spectra help astronomers understand more than just redshift. They reveal the galaxy’s age, star formation rate, and overall composition. This broad view is vital for mapping the universe's expansion and for piecing together the story of the cosmos.