ASTR 1 Midterm 2 Fall 2026

What is the name of layer 1?

Core, 10^7K center where fusion happens

What is the name of layer 2?

Radiation Zone, energy is transported upward by photons from the core

What is the name of layer 3?

Convection Zone, boiling plasma as everything rises

What is the name of layer 4?

subsurface flows (not studied in lecture), currents of plasma just under the surface that transports the magnetic field and affects the solar cycle

What is the name of layer 5?

Photosphere, 6000k and top of boiling water, most of our interactions with the sun

What is the name of layer 6?

chromosphere, hotter than the photosphere at 10^4-10^5k, middle layer of solar atmosphere

What is the name of layer 7?

Corona, 10^k and outermost layer of the solar atmosphere

Sun’s luminosity and size

3.8×10²6 watts and 10x diameter of Earth

luminosity

energy radiated / time

fusion

combines 2 nuclei to make bigger nuclei, only happens in the core of a star

How does the sun maintain its structure?

fight between gravity and pressure from nuclear fusion

Solar Thermostat

decline in core temp = fusion rate drops and core contracts and heats

rise in core temp = fusion rate rises and core expands and cools

helioseismology

measuring of patterns of vibration on surface of the sun

neutrinos

antimatter created during fusion that interacts very little with matter so it can fly straight through the sun

sun spots

cooler areas (4000k) on the surface of the sun that are formed because of a strong magnetic field

solar flares

loops connecting sun spots break because of the sun’s movement, bursts of x-rays

corona mass ejection

10^13-10^14kg material is released out into the solar system

Carrington Event

massive coronal mass ejection

solar cycle

every 11 years number of sun spots rises and falls

differential rotation

different parts of a rotating object move a different angular velocities based on different latitudes, depths, or times

4H to 1 He

fusion of Helium, .7% less energy than 4H, released energy stops the sun from collapsing

parallax

for nearby stars, the measurement of the separation between 2 points to solve for distance

parsecs (d)

d=1/p(in arcseconds)

the 3 observational categories

imaging, spectroscopy, time monitoring

apparent magnitude

magnitude of a star as we see it

What 2 factors are most important in determining the current appearance of a star?

mass and stage of life

which units are appropriate for measurement of apparent brightness?

watts per square meter

how did astronomers discover the relationship between spectral type and mass for main sequence stars?

they measured masses and spectral types of main sequence stars in binary systems

apparent brightness

how bright a star is to an observer, dependent on luminosity and distance

spectroscopic binary

determining a pair of stars orbit eachother through Doppler Shifts

interstellar medium

the gas and dust that lies between the stars in the Milky Way

what must we measure to infer a star’s surface temperature?

spectral type

what do we need to calculate luminosity?

apparent brightness, distance, and the assumption that there is no interstellar gas absorbing or scattering light between us and the star

How do we determine a star’s mass directly?

by a binary star

visual binary

a pair of stars that we see directly orbiting eachother

eclipsing binary

pair of stars that orbit each other in the plane of our line of sight

where do stars spend most of their time?

main sequence

the higher the mass…

the higher the luminosity and the shorter the life

the lower the mass…

the lower the luminosity and the longer the life

open cluster

newly formed stars loosely packed together (pleiades)

globular cluster

millions of stars held in a dense ball and some of the oldest structures in the universe

main sequence turnoff

marks where starts stop fusing Hydrogen and starts its evolution into the Giant phase

pulsating variable stars

changes in brightness of stars over a period of time

Cepheid variables

most luminous pulsating stars, 1000 times brightness of sun

molecular clouds

cold and dense interstellar clouds where stars are born

interstellar reddening

change in color of starlight as it passes through dusty gas

gravitational equilibrium

outward push of gas pressure balances inward pull of gravity

thermal pressure

increase of pressure as temperature rises, happens in interstellar clouds after collapsing

oldest stars in the universe

born of just Hydrogen and Helium, more massive stars because clouds could not cool as much as today’s

how does a cloud become a star?

as cloud becomes smaller and denser, thermal energy is created via friction and cannot escape as easily

protostar

forming star that cannot yet fuse Hydrogen

what would happen to the surface temperature if a star’s radius increased by 7 times but there was no change in luminosity?

the star’s surface temperature would decrease

population 3 stars

first stars to form, born only about 150 million years after Big Bang

circumstellar disk

material collapses on protostar, jets push material out but angular momentum forms this as it heats up and speeds up

what would happen to a protostar that formed without any rotation?

there would be no planets. the rotation that creates the circumstellar disk is what forms planets

development of a protostar

1. luminosity and temp grows as matter collects

2. surface temp remains near 3000k while convection is main energy transport

3. luminosity remains nearly constant during late stages of contraction while radiation transports energy

4. core temp continues to rise until star begins fusion

what would happen to a contracting cloud fragment if it were not able to radiate away its thermal energy?

internal pressure would increase

Pauli Exclusion Principle

electrons in gas or plasma sort themselves into different energy states

example: students finding seats in a lecture (students electrons and seats energy level)

what happens in a low-mass star when core temp rises enough for helium fusion to begin?

Helium fusion rises quickly

what happens when the star’s core runs out of helium?

Helium fuses in a shell around the core

what happens when a star runs out of hydrogen to fuse?

the core of the star contracts under gravity’s pressure, which rises temperatures and starts Helium fusion where then it expands into the red giant phse

life of a low-mass star

0. star formation

1. protostar

2. main-sequence (90% of life)

3. red giant

4. He burning star

5. 2nd red giant

6. planetary nebula

7. white dwarf

planetary nebula

envelop lifts and core is revealed