Astronomy: Light, Telescopes, Stars, and Cosmology

Vocabulary flashcards covering electromagnetic radiation, spectroscopy, telescope optics, stellar classification, stellar lifecycles, relativity, and cosmology.

Electromagnetic Radiation

Energy consisting of oscillating electric and magnetic fields perpendicular to each other and to the direction of propagation, traveling at approximately $300\text{ million meters per second}$ in a vacuum without requiring a material medium.

Photon

A discrete particle-like packet of electromagnetic energy that exhibits both wave-like and particle-like properties.

Radio Waves

The type of electromagnetic radiation possessing the longest wavelengths and lowest photon energies, typically used to carry signals for radio, television, and communications.

Microwaves

Electromagnetic radiation with shorter wavelengths and higher photon energies than radio waves, commonly used in radar, communications, and microwave cooking.

Infrared Radiation

Electromagnetic radiation falling between microwaves and visible light, emitted by warm objects and utilized in thermal imaging.

Visible Light

A narrow band of electromagnetic radiation between infrared and ultraviolet that is detectable by human eyes and used for vision and illumination.

Ultraviolet Radiation

Electromagnetic radiation having shorter wavelengths and higher photon energies than visible light, capable of causing sunburn and used for sterilization and curing materials.

X-rays

High-energy electromagnetic radiation with very short wavelengths, commonly used for medical imaging and examining materials.

Gamma Rays

Electromagnetic radiation possessing generally the shortest wavelengths and highest photon energies, used in cancer treatment and for studying energetic astronomical processes.

Blackbody

An idealized physical object that absorbs all electromagnetic radiation incident upon it without reflection, emitting thermal radiation with a continuous spectrum determined entirely by its temperature.

Spectroscopy

The analysis of light separated by color into a spectrum to determine an astronomical object's temperature, chemical composition, line-of-sight motion, and atmospheric properties.

Doppler Effect (in Astronomy)

The shift in spectral line wavelengths caused by relative line-of-sight motion; a blueshift indicates motion toward Earth, while a redshift indicates motion away from Earth.

Continuous Spectrum

A smooth rainbow containing all wavelengths within an observed range without any gaps, produced when a hot, dense source emits light across a broad range of wavelengths.

Light-Gathering Power

The capability of a telescope to collect light to detect faint objects, determined by the surface area of the primary mirror or lens and proportional to $\text{diameter}^2$.

Resolving Power

The capability of a telescope to reveal fine spatial detail, determined by aperture diameter and observing wavelength, with larger apertures and shorter wavelengths yielding finer resolution.

Atmospheric Turbulence

The movement of air layers that bends incoming light unevenly, causing celestial objects to twinkle and blurring telescopic images regardless of mirror size.

Light Pollution

The brightening of the night sky caused by artificial city light scattering through the atmosphere, which conceals faint celestial objects.

Refracting Telescope

A telescope design that uses an objective lens mounted at the front to bend (refract) and focus incoming light.

Chromatic Aberration

An optical distortion in simple lenses where different wavelengths of light are focused at different focal points, creating colored fringe artifacts around images.

Reflecting Telescope

A telescope design that focuses light by bouncing it off a curved primary mirror, eliminating chromatic aberration and allowing mechanical support across the back.

Hertzsprung–Russell (H–R) Diagram

A graph that plots stellar luminosity against surface temperature or spectral class, grouping stars into distinct regions such as the main sequence, giants, supergiants, and white dwarfs.

Main Sequence

The prominent diagonal band on an H–R diagram comprising stable stars fusing hydrogen into helium in their cores while maintained in hydrostatic equilibrium.

Hydrostatic Equilibrium

The stable physical balance within a star where the inward pull of gravity is precisely counteracted by outward thermal and radiation pressure.

Main-Sequence Turnoff

The point on a star cluster's H–R diagram where stars begin exhausting core hydrogen and leave the main sequence, serving as an indicator of the cluster's age.

Trigonometric Parallax

A direct geometric technique for measuring stellar distance based on the apparent angular shift of a nearby star against distant background stars as Earth orbits the Sun.

Luminosity

The total amount of electromagnetic energy emitted by a star per second into space.

Apparent Brightness

The amount of stellar energy received per second per unit area as measured at Earth.

Protostar

An early evolutionary stage formed when a dense, cold pocket of a molecular cloud collapses and heats up before the ignition of sustained core hydrogen fusion.

White Dwarf

A hot, dense, roughly Earth-sized stellar remnant left behind by a low-to-intermediate-mass star, supported against gravitational collapse by electron degeneracy pressure.

Neutron Star

An extremely dense, city-sized stellar remnant composed primarily of closely packed neutrons, formed from the core collapse of a massive star.

Black Hole

A region of spacetime formed by complete gravitational collapse beyond the support limit of matter, surrounded by an event horizon from which no light can escape.

Event Horizon

The boundary surrounding a black hole where the gravitational escape velocity equals the speed of light.

Escape Velocity

The minimum speed required for an unpropelled object to break free from the gravitational pull of a massive body.

Type Ia Supernova

A cataclysmic thermonuclear explosion triggered by runaway carbon fusion in an accreting or merging white dwarf in a binary system, generally destroying the white dwarf entirely.

Type II Supernova

A core-collapse explosion of a massive star that still retains its outer hydrogen envelope, leaving behind a neutron star or black hole.

Gravitational Redshift

The shift of electromagnetic waves toward longer wavelengths as they escape a gravitational well, consistent with general relativity's prediction of gravitational time dilation.

Spacetime

The unified four-dimensional geometric fabric combining space and time, whose curvature by mass and energy constitutes gravity according to general relativity.

Barred Spiral Galaxy

A spiral galaxy characterized by a central bar-shaped band of stars crossing the nucleus, with spiral arms originating from the ends of the bar; the Milky Way is classified as this type.

Elliptical Galaxy

A galaxy presenting a smooth, round or oval shape without spiral arms, composed predominantly of older stellar populations with minimal cold gas and little active star formation.

Irregular Galaxy

A galaxy lacking structured spiral or elliptical symmetry, typically rich in interstellar gas and undergoing active star formation.

Cepheid Variable Star

A pulsating star whose period of brightness variation correlates directly with its intrinsic luminosity via the period–luminosity relationship, enabling extragalactic distance determination.

Hubble's Law

The linear relationship $v = H_0 d$, stating that the recessional velocity ($v$) of distant galaxies is directly proportional to their distance ($d$) due to cosmological expansion.

Cosmic Microwave Background (CMB)

An isotropic, nearly uniform electromagnetic bath exhibiting a blackbody spectrum at approximately $2.73\,K$, representing remnant radiation from roughly $380{,}000\text{ years}$ after the Big Bang.

Absorption Spectrum

Bright colored lines against a dark background.

Emission Spectrum

A continuous rainbow crossed by dark lines.

Light-gathering power

Detecting fainter objects

  Explain why the largest telescopes are reflecting and not refracting.

• Larger mirrors are easier to support

• Mirrors reflect and bounce off light than go through

• Can be segmented

• Avoids color separation

What is the spectral class order?

O, B, A, F, G, K, M (Oh Being A Fat Gay King Might)

Describe two methods for determining stellar distances, Method 1:

• Trigonometric parallax

  • As Earth orbits the Sun, a nearby star appears to shift against more distant background stars. A larger parallax shift means a closer star.

Describe two methods for determining stellar distances, Method 2:

• Spectroscopic distance estimation

1. Analyze the star’s spectrum to determine its spectral type and luminosity class.

2. Use its classification and a calibrated H–R diagram to estimate its luminosity.

3. Compare that luminosity with its apparent brightness to calculate distance.

Spica Star

Blue; very hot

B-type star

Upper left: hot and highly luminous

Rigel Star

Blue-white; hot

Blue supergiant

Near the top, toward the left

Betelgeuse Star

Red; cool surface

Red supergiant

Upper right: cool but extremely luminous

Antares Star

Red; cool surface

Red supergiant

Upper right, near the same general region as Betelgeuse

Arcturus Star

Orange; cooler than the Sun

Giant

Above and right of the Sun

Aldebaran Star

Orange-red; cool

Giant

Upper-right giant region, below the supergiants

Sirius A Star

White; hotter than the Sun

Main-sequence star

On the main sequence, above and left of the Sun

Vega Star

White to blue-white; hotter than the Sun

Main-sequence star

On the main sequence, above and left of the Sun

Our Sun Star

Yellow; intermediate temperature

Main-sequence star

On the main sequence at one solar luminosity

How do stars live and die?

• Form when gravity causes a dense region of a cold molecular cloud—mostly gas with some dust—to collapse. The collapsing material heats up, forming a protostar. When its core becomes hot enough for sustained hydrogen fusion, it becomes a main-sequence star

How do clusters stars help our stellar evolution?

• Lets astronomers compare how mass affects evolution without confusing it with large differences in age.

Discuss three pieces of evidence that support the theory of relativity, 1:

Mercury’s orbital precession

Mercury’s orbit changes orientation slightly more than Newtonian gravity alone predicts.

General relativity accounts for the extra 43 arcseconds per century.

Discuss three pieces of evidence that support the theory of relativity, 2:

Bending of light

Light passing near a massive object changes direction, producing effects such as gravitational lensing.

Light follows paths through curved spacetime.

Discuss three pieces of evidence that support the theory of relativity, 3:

Gravitational redshift

Light escaping a gravitational field is observed at a longer wavelength.

This agrees with relativity’s prediction that clocks deeper in a gravitational field run more slowly relative to distant clocks.

Describe the three attributes a hypothesis must possess.

• Explanatory: It proposes a natural explanation for observations.

• Testable: It makes predictions that observations or experiments can check.

• Falsifiable: There must be a possible observation that would show it is wrong.

Provide and explain evidence that the universe is expanding

• v=H0​d

  • Here, v is recession velocity, d is distance, and 0 is the present-day Hubble constant. The pattern is what we expect when distances between widely separated, unbound galaxies increase as space expands.

Showing Hubble’s constant and describe how we infer the age of the universe from this diagram.

• A steeper slope means a larger H0​, implying a shorter expansion timescale.

• A shallower slope means a smaller H0​, implying a longer expansion timescale

Constellation Draco:

A long, winding chain of stars ending in a small, four-star diamond or quadrilateral head

Northern sky, below and around the Little Dipper; the head is very low near the north–northeast horizon

Find Polaris and the two Dippers first. Body winds between them. Mountains may hide its low head.

Constellation Leo:

A backward question mark, called the Sickle, joined to a triangle marking the hindquarters

Eastern sky, rising; the head is higher than the tail

Regulus marks the bottom of the Sickle; Denebola marks the tail.

Constellation Orion

A large hourglass or bow-tie outline with three closely spaced stars in a straight line across its middle

Southern sky, slightly west of due south

Find the three Belt stars first. Reddish Betelgeuse and blue-white Rigel mark opposite corners.

Constellation Gemini

Two roughly parallel chains of stars, with two bright stars marking the heads

High in the southeast, approaching the southern sky

Castor and Pollux are the bright pair. Gemini lies above and east of Orion.

Constellation Perseus

A branching, somewhat curved Y-shaped pattern; drawings vary

High toward the west–northwest, below and west of the overhead region

Find Cassiopeia’s W, then look toward the region between Cassiopeia and Taurus. Mirfak is its brightest star; Algol lies on a side branch.

Light spreads out, and therefore…

Becomes fainter further away

What are Bluetooth and WiFi placed on an electromagnetic diagram?

They use radio waves

Name one difference between X-rays and yellow light

Wavelengths

Which spectrum corresponds to daylight?

Big and bulky one

The Voyager missions have gone further into space than anything else. How far have they gone?

Edge of the solar system

Are the biggest telescopes reflecting or refracting?

Reflecting

Where is Hubble?

Just above the Earth

Which element is important for stars?

Hydrogen

Can we use the parallax method to measure distance to Mars?

Yes

Based on Betelgeuse and Rigel, what would you expect the temperature of the Sun’s surface to be?

5,000-7,000 K

Luminosity and brightness, what does it depend on?

The star’s size, temperature, distance, and luminosity

Star A and Star B have the same parallax. If the brightness of Star A is larger than that of Star B, then…

Star A is more luminous than Star B

What does the H-R Diagram plot?

Spectral Class & Luminosity

What if fusion suddenly halted?

The star would collapse

Our star (G2) will die; what type of death?

Low-mass planetary nebula & white dwarf

What would determine whether a star ends as a black hole?

Mass

Close to a black hole, what is the escape velocity?

10 billion mph

What are gravitational waves?

Distortions in spacetime

Can we use the parallax method to find the distances of stars?

No

Given that a type II supernova is the death of a massive star, where in a spiral galaxy would you look for one?

Spiral Arms

• Suppose Hubble’s constant was smaller. The universe would be __:

Older than we thought

Which has the most resolving power?

An 8-inch refracting telescope

The Sun may explode in a type I supernova

No, it is not in a binary system

Do medium-mass stars go through the red giant stage?

Yes

Hubble’s constant is 60 km/s/Mpc. Suppose we learn it's 45; the age of the universe will be __ than we thought

Older