PHYS 3022 Lecture 1-3 (Wise)

A self-gravitating celestial object, in which there is or there once was sustained thermonuclear fusion of hydrogen in the core.

Star

These are the primary source of energy in the universe.

Stars

Stars form following the gravitational collapse of this.

A cold molecular cloud

If the mass of a collapsing cold molecular cloud is more than ___ of the mass of the Sun, the central temperature will be higher than the threshold for hydrogen fusion, leading to star formation.

8%

Astrophysics, along with particle physics, uses cgs units, which are based on the:

centimeter, gram, and second

When equilibrium is reached between the inward pull of gravity and the outward pressure from the hot gas and thermonuclear reactions in the core of a star.

Hydrostatic equilibrium

Nuclear energy generated in the core of a star is approximately equal to the:

energy radiated from the surface

A star at the end of its formation stage. It will remain in this state for most of its life.

Main sequence star

Based on the Big Bang theory, only these three elements were produced in the early universe. All other elements were produced in stars and supernovae.

Hydrogen, helium, trace lithium

In astrophysics, we refer to all elements heavier than helium as ___ for historical reasons.

metals

Thermonuclear reactions in stars which are responsible for the synthesis of various elements, such as carbon, oxygen, silicon, and iron.

Nucleosynthesis

All elements heavier than iron are produced in these.

Supernovae

The explosive deaths of massive stars.

Supernovae

The most direct method of measuring the distance to stars, which is the apparent shift in the position of a star against the background of more distant stars as the Earth orbits the Sun.

Stellar parallax

This is defined as half the angular shift observed over a six-month period.

Parallax angle

Because the parallax angles are very small, they are usually measured in this dimension.

Arcseconds (")

A star with a parallax angle of 1 arcsecond is at a distance of ___ parsec(s).

1

The nearest star to the Sun.

Proxima Centauri

The total amount of radiation energy received per unit area per unit time, including light from all wavelengths.

Radiation flux F

Magnitude measurements of stars use this scale.

Inverse logarithmic

The reference fluxes are chosen such that this star has an apparent magnitude of m = 0 in all bands.

Vega

Apparent magnitude increasing implies that brightness:

decreases

The "apparent magnitude" an object would have if it were located at a standard distance of 10 parsecs.

Absolute magnitude M

The absolute magnitude is only dependent on this characteristic of an object.

Intrinsic luminosity

The quantity (m - M) is called this.

Distance modulus

The mass of the Sun, usually reported in grams.

Solar mass

The radius of the sun, usually reported in centimeters.

Solar radius

The luminosity of the sun, usually reported in erg/s.

Solar luminosity

High-energy packets of two protons and two neutrons bound together, making them identical to a helium-4 nucleus.

Alpha particles

1 AU is measured as the:

mean distance between the Sun and Earth

The brightest stars have a magnitude of:

1

The dimmest visible stars have a magnitude of:

6

Radiation is quantized in these, which are massless particles that travel at the speed of light c.

Photons

A momentum transfer when photons are absorbed or reflected.

Radiation pressure

Wavelength is ___ proportional to energy.

inversely

This spectral region contains wavelengths less than 0.1 Angstroms.

Gamma rays

This spectral region contains wavelengths from 0.1-100 Angstroms.

X-rays

This spectral region contains wavelengths from 100-4000 Angstroms.

Ultraviolet

This spectral region contains wavelengths from 4000-7000 Angstroms.

Visible (optical)

This spectral region contains wavelengths from 7000 Angstroms to 1 mm.

Infrared

This spectral region contains wavelengths that are greater than 1 mm.

Radio

A physical entity that absorbs all radiation that falls upon it.

Blackbody

What are the seven variables that radiation is dependent on?

Position, direction, time, energy

When a system has a uniform temperature T and emits radiation isotropically.

Thermal equilibrium

The energy emitted per unit area per unit time per unit solid angle per unit frequency, resulting in units of erg/s/Hz/cm^2/sr, where sr is a steradian.

Specific intensity

How many steradians are there in a sphere?

4π

This states that the flux of a blackbody is proportional to the fourth power of its temperature.

Stefan-Boltzmann law

As star temperature increases, the flux:

increases rapidly

Massive stars have higher temperatures, and therefore ___ fluxes, than low-mass stars.

higher

Due to large stars' higher luminosities, they have ___ lifetimes.

shorter

Hotter stars have their peak emission at ___ wavelengths, which is why they appear bluer.

shorter

Cooler stars have their peak emission at ___ wavelengths, which is why they appear redder.

longer

The peak of the blackbody spectrum shifts to ___ wavelengths as the temperature increases.

shorter

When something has physical properties that are the same in all directions.

Isotropic

The study of radiation inside a certain range of wavelength.

Photometry

The magnitude measured over all wavelengths.

Bolometric magnitude

To obtain the flux for a given photometric band, we use a filter in a telescope that is ___ to the radiation in this waveband.

nearly transparent

We can use the ratio of fluxes in different filters to estimate the ___ uniquely for a blackbody.

effective temperature

The magnitude difference between two filters.

Color

If B-V is negative, is the star bluer or redder?

Bluer

Lower magnitudes represent ___ objects.

brighter

Color indices only depend on the ___ of the star.

temperature

The magnitude considering the entire spectrum.

Bolometric magnitude

The difference between a star's bolometric magnitude and visual magnitude.

Bolometric correction

The fraction of flux detected at a given wavelength.

Sensitivity function

The flux in wavebands (filters)

Photometry

The study of spectral features.

Spectroscopy

Spectroscopy was pioneered by this scientist in 1814, when he observed the Sun and found spectral lines. These solar spectral lines are now named after him.

Frauhofer

Each of these solar spectral lines is associated with a certain element or molecule. Lines are caused by absorption of stellar radiation by atoms and molecules in the stellar atmosphere.

Frauhofer lines

This stellar spectra classification scheme labels spectra with capital letters according to the strength of hydrogen absorption lines, beginning with A for the broadest lines.

Pickering & Fleming

This stellar spectra classification scheme was created when Annie Jump Cannon rearranged a previous scheme to classify stars according to their color and thus temperature.

Harvard classification scheme

The Harvard sequence of temperature classifications, running from the hottest blue star to the coolest red star.

OBAFGKM

The beginning of the Harvard sequence has ___ type stars (such as O-type).

early

The end of the Harvard sequence has ___ type stars.

late

The Harvard star classes are further divided by number with the ___ stars having lower numbers.

hotter

If a star has a negative or very small B-V color index, what does this conceptually tell us about the star's physical properties?

It's relatively hot and blue

This is by far the most abundant element in stellar atmospheres.

Hydrogen

Hydrogen Balmer absorption lines reach their maximum strength in these stars.

A

The hottest O-type stars are so energetic that almost all of their Hydrogen is:

completely ionized

This type of study looks at detailed structures through their spectral lines.

Spectroscopy

The flux over all wavelengths.

Bolometric flux