Stars & HR Diagram
What two properties are plotted on a Hertzsprung-Russell (H-R) diagram?
Temperature and Luminosity
What causes the change in seasons on Earth?
The tilt of Earth’s axis and Earth’s orbit around the Sun cause the angle and intensity of sunlight to change, producing seasons.
What is a galaxy?
A massive system of stars, gas, dust, and dark matter bound by gravity.
What is the difference between a star and a planet?
Star: luminous ball of plasma producing its own light via fusion. Planet: non-luminous body orbiting a star.
Why do we have day and night on Earth?
Earth rotates on its axis; the side facing the Sun experiences day, the opposite side night.
On the H-R diagram, where are the main sequence stars located relative to giants and white dwarfs?
Along a diagonal band from hot/luminous (upper-left) to cool/dim (lower-right); giants are above/right and white dwarfs below/left.
During which month does the Northern Hemisphere experience the summer solstice and why?
June (around June 20–22) — Northern Hemisphere is tilted most toward the Sun, receiving most direct sunlight (summer solstice).
Name the three main shapes of galaxies.
Spiral, elliptical, and irregular.
Name one piece of evidence that supports the Big Bang theory.
Cosmic microwave background radiation, observed redshift of distant galaxies, abundance of light elements.
What causes the phases of the Moon?
Moon phases result from the Moon’s position relative to Earth and Sun, showing different portions of its illuminated half.
What color (temperature) and luminosity characteristics identify a red giant on the H-R diagram?
Red giants are cool (red/orange), very luminous, located in the upper-right region.
Explain why temperatures in summer are warmer even though Earth is sometimes farther from the Sun.
Because tilt causes more direct sunlight and longer daylight hours in summer, Earth’s surface absorbs more energy even if Earth’s distance varies slightly.
Which type of galaxy is the Milky Way, and what feature identifies that type?
The Milky Way is a spiral galaxy; it has a central bulge with spiral arms.
What is a nebula and what role does it play in star formation?
A cloud of gas and dust; nebulae are regions where dense pockets collapse under gravity to form protostars.
How does the tilt of Earth’s axis relate to seasons?
Earth’s tilted axis means different hemispheres receive more direct sunlight at different times of year, producing seasons.
Explain how you would use an H-R diagram to estimate a star’s relative age or evolutionary stage.
Compare the star’s position to main sequence, giant, or white dwarf regions; location indicates stage (e.g., main sequence = hydrogen fusion; giant = post–main-sequence).
Describe how daylight hours change from winter to summer in the Northern Hemisphere and why.
Daylight hours increase from winter to summer: winter has shorter days and longer nights; summer has longer days and shorter nights because the hemisphere is tilted toward the Sun.
How can astronomers tell that galaxies are moving away from us? Name the evidence.
By observing redshift of spectral lines (Doppler effect) showing galaxies’ light shifted toward longer wavelengths; Hubble’s law.
Explain how different wavelengths (such as visible light vs. radio waves) help astronomers learn about the universe.
Different wavelengths reveal different processes and objects (e.g., radio reveals cold gas and pulsars; X-rays reveal high-energy events; visible shows stars and galaxies)
Explain how the positions of the Moon and Sun together produce spring tides.
Spring tides occur at new and full moon when gravitational pulls of Sun and Moon align, increasing tidal range.
Given a star with high luminosity and high surface temperature, where would it fall on the H-R diagram and what is its likely spectral class?
Upper-left region (very luminous, hot) — likely an O or B spectral class (massive, young).
Compare and contrast the axial tilt explanation of seasons with the incorrect idea that seasons are caused by Earth being closer to or farther from the Sun.
Axial tilt explanation: seasons result from tilt changing sunlight angle and day length. Incorrect distance idea: Earth's orbit is nearly circular, so distance changes are too small to cause seasons; tilt is the dominant factor.
Describe one observational difference between an elliptical galaxy and a spiral galaxy in terms of star populations and appearance.
Elliptical: older, redder stars, little gas/dust, smooth oval shape. Spiral: mixed ages including young blue stars in arms, lots of gas/dust, defined spiral structure.
Outline how scientific data are used as evidence to develop theories about the origin of the universe (brief, 2–3 sentences).
Scientists collect observations (light spectra, cosmic background, element abundances), test models (expansion, nucleosynthesis), and refine theories when predictions match data.
Describe how the relative distances of the Sun and other stars explain why the Sun appears much brighter in our sky than other stars.
The Sun is much closer to Earth than any other star (so it appears much brighter), even though many stars are much more luminous intrinsically.