From the Distant Past Exhibit

Spectra: a Closer Look
What Are Spectra?

Hubble is famous for its stunning images of the universe. But many of Hubble’s most amazing discoveries come from its observations of cosmic objects’ spectra.


White light is made of colors. The light from a cosmic object is dispersed by Hubble’s instruments into its component rainbow of colors.

laser art plot

Astronomers measure the brightness of the light across the colors to create a plot of the spectrum. Objects beyond the solar system are too far away to visit, but astronomers can learn all kinds of characteristics and details by studying these plotted lines.

Reading a Spectrum

Spectra's shapes reveal the hidden secrets of astronomical objects. Sometimes astronomers can learn much more information about an object using these simple lines than they can from a picture.


A hot, young star will produce a relatively smooth spectrum.


This star is heating the cloud of hydrogen gas that surrounds it. Astronomers looking at the spectrum produced by this starlight and surrounding gas will see peaks at specific wavelengths that indicate the presence of energized hydrogen.


This star's light passes through a cloud of cold hydrogen gas. The gas acts like a filter, absorbing portions of the light. Astronomers looking at this spectrum will identify these dips at specific wavelengths to detect the presence of the cold cloud.

laser art sample

The universe is a complex place, and real spectra are quite intricate, revealing information about a wide variety of an object's characteristics.

Why Look at Spectra?

We can learn much about cosmic objects by looking at images, but even more by looking at their light. So we use instruments like spectrographs that break light into colors, revealing hidden characteristics.

hot vs. cool curve shape

The shape of a star’s spectrum reveals its temperature. Spectra that peak near the blue end of the scale indicate young stars that burn hot. Spectra that peak closer to the red end of the scale indicate stars with cooler temperatures.

Hot stars burn through their fuel quickly, dying in less than 50 million years. (Our Sun, in contrast, will burn for about 10 billion years.) So astronomers know that when they find a hot, blue star, that star is always young. Astronomers can classify entire galaxies by whether they contain mostly blue stars or mostly red stars, and they can trace the history of galaxies by the studying combinations of stars they contain.

peaks of different elements

Peaks and dips at specific points in a spectrum indicate the presence of elements like carbon, oxygen, hydrogen, iron, and more. That information can be used to make other conclusions about the object. Stars that contain just hydrogen and helium, for example, formed early in the universe. Stars with heavier elements formed later, after their predecessors exploded and seeded the universe with those elements.

Shortly after the Big Bang, nearly all the universe’s matter was made up of light elements, particularly hydrogen and helium. The earliest stars created the heavier elements – like carbon, nitrogen, oxygen, and iron – in the cores. By studying the spectra of the stars and the gas between the stars, we can learn how the universe evolved from the Big Bang to the present. We can also use spectra to study the composition of planets around other stars for components like water, which is essential for the rise of life.

wavelength shift

Astronomers know where peaks and dips should be located in a spectrum. When those lines appear shifted, it means the object is in motion. The spectrum can be used to identify the object’s velocity and whether it’s moving toward us or away.

The ability to learn the velocity of orbiting objects allows us to determine mass. We can use the information gleaned from spectra to find the mass of the Sun, other stars, black holes, galaxies, and even clusters of galaxies. Determining the velocity of galaxies has allowed us to discover that the universe is expanding, that a strange dark energy appears to be speeding the expansion, and that we are surrounded by invisible dark matter.

See the Field Guide to Cosmic Spectra

From the Distant Past Event Page