Free DSST Astronomy Practice Test (2026)

Question 1

An asteroid orbits the Sun with an average distance (semi-major axis) of 4 AU. What is its orbital period?

Accepted Answer

8 years

Answer

4 years

Answer

16 years

Answer

2 years

Question 2

Star A looks blue-white and star B looks red. What can you conclude about star A compared with star B?

Accepted Answer

It has a higher surface temperature

Answer

It has a lower surface temperature

Answer

It is necessarily closer to Earth

Answer

It is necessarily moving toward Earth

Question 3

Earth's seasons are caused mainly by

Accepted Answer

the tilt of Earth's axis, which changes sunlight angle and day length

Answer

the change in Earth's distance from the Sun over its orbit

Answer

small changes in the amount of energy the Sun gives off

Answer

the Moon's changing position and pull on Earth's oceans

Question 4

Why did the giant planets form in the outer solar system rather than near the Sun?

Accepted Answer

Beyond the frost line, ices could freeze, giving extra solid material for big cores

Answer

The outer solar nebula was hotter, so gas collected there more easily

Answer

The Sun's gravity pushed heavy elements outward

Answer

The giant planets formed around other stars and were later captured

Question 5

Sunspots look dark in images of the Sun because they are

Accepted Answer

cooler than the surrounding photosphere

Answer

holes that reveal the Sun's cooler interior

Answer

shadows cast by planets crossing the Sun

Answer

dense clouds of gas floating above the surface

Question 6

Henrietta Leavitt found that Cepheid variable stars with longer pulsation periods are more luminous. This discovery allowed astronomers to

Accepted Answer

measure distances to other galaxies

Answer

measure the masses of binary stars

Answer

determine the chemical makeup of stars

Answer

find the ages of globular clusters

Question 7

A cool, thin gas lies between an observer and a hot source that gives off a continuous spectrum. What kind of spectrum does the observer see?

Accepted Answer

A continuous spectrum crossed by dark absorption lines

Answer

Only bright emission lines on a dark background

Answer

A smooth continuous spectrum with no lines at all

Answer

Only radio waves, because the gas blocks visible light

Question 8

If the distance between two bodies is tripled, the gravitational force between them becomes

Accepted Answer

one-ninth as strong

Answer

one-third as strong

Answer

three times as strong

Answer

one-sixth as strong

Question 9

The Kepler space telescope found thousands of exoplanets by detecting

Accepted Answer

small, regular dips in a star's brightness as a planet crosses in front of it

Answer

back-and-forth shifts in the lines of a star's spectrum

Answer

direct photographs of planets next to their stars

Answer

brief brightening of a background star by a foreground planet

Question 10

A lunar eclipse can occur only when the Moon is at which phase?

Accepted Answer

Full moon

Answer

New moon

Answer

First quarter

Answer

Waning crescent

Question 11

A star of about 20 solar masses will most likely end its life by

Accepted Answer

exploding as a supernova and leaving a neutron star or black hole

Answer

shedding a planetary nebula and leaving a white dwarf

Answer

cooling slowly into a brown dwarf

Answer

fusing hydrogen steadily forever

Question 12

In Copernicus's Sun-centered model, how is the apparent backward (retrograde) motion of Mars explained?

Accepted Answer

Earth, moving faster on a smaller orbit, overtakes Mars

Answer

Mars travels on a small circle called an epicycle

Answer

Mars actually reverses direction in its orbit

Answer

Earth's daily spin on its axis makes Mars seem to loop

Question 13

According to Hubble's law, a galaxy twice as far away as another galaxy is

Accepted Answer

moving away about twice as fast

Answer

moving away at the same speed

Answer

moving away half as fast

Answer

moving toward us twice as fast

Question 14

If a star's surface temperature doubled while its radius stayed the same, its luminosity would increase by a factor of

Accepted Answer

16

Answer

2

Answer

4

Answer

8

Question 15

Venus has the hottest surface of any planet, even though Mercury is closer to the Sun. Why?

Accepted Answer

Its thick carbon dioxide atmosphere traps heat in a runaway greenhouse effect

Answer

It spins very fast, so friction heats its surface

Answer

Its core is still producing energy by nuclear fusion

Answer

Its clouds absorb almost all the sunlight that reaches them

Question 16

In an elliptical orbit, Kepler's second law says a planet moves

Accepted Answer

fastest when it is closest to the Sun

Answer

at the same speed all the way around

Answer

fastest when it is farthest from the Sun

Answer

fastest at the two ends of the orbit's short axis

Question 17

Compared with spiral galaxies, elliptical galaxies typically have

Accepted Answer

little gas and dust, mostly old stars and little new star formation

Answer

large amounts of gas and many young blue stars

Answer

prominent spiral arms with bright star-forming regions

Answer

a thin, flat disk that rotates in an orderly way

Question 18

A star is cool and red but extremely luminous. What must be true of it?

Accepted Answer

It has a very large radius

Answer

It has a very small radius

Answer

It is very close to Earth

Answer

It is a young star still contracting

Question 19

Most long-period comets are thought to come from

Accepted Answer

the Oort cloud, a distant shell of icy bodies

Answer

the asteroid belt between Mars and Jupiter

Answer

the ring systems of Saturn and Uranus

Answer

the space between the orbits of Earth and Venus

Question 20

At about what time does a first-quarter Moon rise?

Accepted Answer

Around noon

Answer

Around sunset

Answer

Around midnight

Answer

Around sunrise

Question 21

General relativity predicted, and observations during the 1919 solar eclipse confirmed, that

Accepted Answer

starlight passing near the Sun is bent by its gravity

Answer

the speed of light depends on the motion of its source

Answer

the Sun is slowly losing mass to its planets

Answer

planets move in perfect circles around the Sun

Question 22

Why does nuclear fusion in a massive star's core stop releasing energy once the core is made of iron?

Accepted Answer

Fusing iron into heavier nuclei uses up energy instead of releasing it

Answer

Iron is too radioactive to stay in the core

Answer

Iron cannot be compressed by gravity

Answer

The star has run out of hydrogen in its outer layers

Question 23

Measurements of distant Type Ia supernovae in the late 1990s showed that

Accepted Answer

the expansion of the universe is speeding up

Answer

the universe is not expanding at all

Answer

the expansion is slowing down because of gravity

Answer

the universe is contracting toward a 'Big Crunch'

Question 24

The lines in a galaxy's spectrum appear at longer wavelengths than the same lines measured in a laboratory. This means the galaxy is

Accepted Answer

moving away from us

Answer

moving toward us

Answer

hotter than the laboratory source

Answer

made of different elements than Earth

Question 25

Compared with the Sun's habitable zone, the habitable zone around a cool, dim red dwarf star is

Accepted Answer

closer to the star and narrower

Answer

farther from the star and wider

Answer

at the same distance from the star

Answer

farther from the star and narrower

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