Unit 16: Stars, Galaxies, and Cosmology
How a star is born, burns, and dies depending on its mass, and how galaxies, an expanding universe, and the Big Bang scale that story up to everything there is.
10 topics · 31 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
How Stars Form From Nebulae
- How it worksHow does a collapsing cloud of gas and dust heat up at its center until nuclear fusion finally ignites?
- How it worksWalk me through a nebula collapsing into a protostar and finally settling into a stable main-sequence star.
- Common mistakeWhy doesn't every collapsing cloud of gas have enough mass to ignite fusion and become a star?
Your turn1 of 2
A cold nebula spends millions of years slowly contracting before its center finally becomes hot enough to ignite fusion. Why does a cold, thin cloud of gas need gravity to pull it together for millions of years before becoming a star?What Powers a Star: Nuclear Fusion
- How it worksHow does outward pressure from nuclear fusion balance the inward pull of gravity to keep a star stable?
- How it worksWalk me through four hydrogen nuclei fusing into one helium nucleus and releasing energy in a star's core.
- Ties togetherHow does the balance between fusion pressure and gravity break down once a star finally runs out of hydrogen fuel?
Your turn1 of 2
A star's size stays remarkably constant for billions of years even though its own enormous gravity is constantly pulling inward. Why can a star shine steadily for billions of years without ever collapsing under its own gravity?The Hertzsprung-Russell Diagram
- How it worksHow does plotting temperature against luminosity sort stars into the main sequence, giants, and white dwarfs?
- Worked exampleFind roughly where a star with a hot blue-white color and very high luminosity would sit on the H-R diagram.
- Common mistakeWhy isn't a red star automatically a cooler, dimmer star than a blue one, once you account for its size?
Your turn1 of 2
A star's luminosity is proportional to its radius squared times its surface temperature to the fourth power. If a star has 7 times the Sun's radius and 1.219 times the Sun's surface temperature, how many times the Sun's luminosity does it have?Main Sequence Stars and Stellar Mass
- How it worksHow does a more massive star's stronger gravity force it to fuse hydrogen far faster, burning through its fuel sooner?
- Worked exampleFind roughly how long a 10 solar mass star lives compared with the Sun's 10 billion year lifespan.
- Ties togetherHow does a star's starting mass on the main sequence determine which path it eventually takes when it dies?
Your turn1 of 2
A star's main-sequence lifetime is roughly proportional to 1 divided by its mass raised to the power 2.5, since more massive stars burn fuel disproportionately faster. If the Sun lives about 10 billion years, about how many billion years would a star with 15 times the Sun's mass live?The Death of Sun-Like Stars: Red Giants and White Dwarfs
- How it worksHow does a dying star's core contracting and heating up push its outer layers outward into a red giant?
- How it worksWalk me through a red giant shedding its outer layers into a planetary nebula and leaving behind a white dwarf core.
- Common mistakeWhy doesn't a white dwarf keep shining through nuclear fusion the way a normal star does?
Your turn1 of 2
Billions of years from now, the Sun will swell to hundreds of times its current size, engulfing the inner planets, before it finally dies. How does a dying star's core contracting and heating up push its outer layers outward into a red giant?The Death of Massive Stars: Supernovae, Neutron Stars, and Black Holes
- How it worksHow does a massive star's iron core suddenly collapsing in seconds trigger a supernova explosion?
- Common mistakeWhy isn't a black hole a cosmic vacuum cleaner that sucks in everything across the galaxy?
- Ties togetherHow does the leftover core's mass after a supernova decide whether it becomes a neutron star or a black hole?
Your turn1 of 2
A massive star can shine steadily for millions of years and then, in the span of seconds, collapse and explode as a supernova visible across an entire galaxy. Why does a star many times the Sun's mass end its life in a catastrophic explosion instead of a gentle fade?Types of Galaxies
- How it worksHow do spiral, elliptical, and irregular galaxies differ in shape, star formation, and how they likely formed?
- Going furtherWhat happens to a spiral galaxy's star-forming gas after it merges with another galaxy?
- Ties togetherHow can two spiral galaxies colliding and merging over billions of years produce a single elliptical galaxy?
Your turn1 of 2
A spiral galaxy is full of young blue stars and visible dust lanes, while a nearby elliptical galaxy glows uniformly with old, reddish stars and almost no gas left. How do spiral, elliptical, and irregular galaxies differ in shape, star formation, and how they likely formed?The Milky Way
- How it worksHow does looking edge-on into our own galaxy's flat spiral disk from inside it create that hazy band we see?
- Worked exampleFind roughly how long it takes light to cross the Milky Way if the galaxy is about 100,000 light-years across.
- Common mistakeWhy isn't the solar system anywhere near the center of the Milky Way galaxy?
Your turn1 of 2
The Milky Way galaxy is about 107857 light-years across. About how many years does it take light to cross the entire galaxy?Evidence for an Expanding Universe: Redshift
- How it worksHow does a galaxy's light stretching to longer, redder wavelengths as it recedes reveal its speed away from us?
- How it worksWalk me through Hubble's observation that more distant galaxies show greater redshift and recede faster.
- Common mistakeWhy doesn't the universe expanding away from us in every direction mean Earth sits at its center?
Your turn1 of 2
Hubble's law states that a galaxy's recession velocity equals the Hubble constant, about 70 kilometers per second per megaparsec, times its distance in megaparsecs. How fast, in kilometers per second, is a galaxy receding if it sits 363 megaparsecs away?The Big Bang Theory and Cosmic Microwave Background
- How it worksHow does running the universe's expansion backward in time point to a single hot, dense beginning?
- How it worksWalk me through the early universe cooling enough, 380,000 years in, to release the light we now see as the cosmic microwave background.
- Common mistakeWhy wasn't the Big Bang an explosion happening at one point inside a pre-existing empty space?
- Ties togetherHow does cosmic microwave background radiation serve as leftover evidence for the Big Bang, detectable in every direction?
Your turn1 of 2
Astronomers describe the Big Bang as the expansion of space itself rather than something happening at one location within space. Why wasn't the Big Bang an explosion happening at one point inside a pre-existing empty space?