Key Concepts in Astronomy and Galaxy Classification

The Hubble classification

divides galaxies into classes according to their morphologies

The nature of 'spiral nebulae'

was observationally confirmed by Edwin Hubble in 1924

White dwarfs

compact stellar remnants resulting from the evolution of low-mass (Sun-like) stars

Supernova remnants

are emission nebulae resulting from supernova explosions

Newton's law of gravity

the gravitational force between two bodies is proportional to the product of the bodies' masses (the bigger the masses, the stronger the force)

Gravitational force acting on a body orbiting the Sun

is directed along the line joining the Sun and the orbiting body

Circular velocity for orbiting the Earth

depends on the mass of the Earth; if the Earth were heavier (while having the same diameter), the circular velocity would be higher

Gravitational acceleration of a body freely falling near the Earth's surface

is independent of the body's mass

Galaxies

systems composed of mainly dark matter, stars and gas, all held together by gravitational forces

The Milky Way Galaxy

a barred spiral galaxy

Supernova remnants typical sizes

have typical sizes of 100,000 light years

Gravitational force dependence on distance

is proportional to the distance separating the bodies (the larger the distance, the weaker the force)

Gravitational force dependence on speeds

depends on speeds of the two bodies - the faster their motion, the stronger their gravitational attraction

Gravitational force decrease with distance

decreases with the 2nd power of the body's distance from the Sun

Circular velocity dependence on orbiting body

depends on the mass of the orbiting body

Gravitational acceleration dependence on Earth's mass

depends on the Earth's mass; if the Earth were more massive (while having the same radius), bodies would be falling faster

Gravitational acceleration dependence on Earth's radius

depends on the Earth's radius; if the Earth were larger (while having the same mass), bodies would be falling more slowly

Gravitational acceleration direction

is directed towards the Earth's center

Galaxies composition

stellar systems; they also contain gas, dust, planets, dark matter and dark energy; dark energy dominates their mass

Milky Way Galaxy size

the second largest and the second most massive galaxy within the Local Group of galaxies

Contributors to the mass of the Universe

helium (¾), hydrogen (¼) and carbon (1%)

Contributors to the mass of the Sun

helium (¾) and hydrogen (¼)

Abundant chemical species in giant planets

methane and ammonia

Abundant chemical elements in Earth's atmosphere

nitrogen and argon

Collision of Milky Way and Andromeda galaxies

they would merge into a single spiral galaxy

Predicted collision time of Milky Way and Andromeda

are predicted to collide in about 4 billion years

Milky Way Galaxy

thought to be surrounded by the Oort Cloud

Oort Cloud

a hypothetical spherically shaped cloud of comets at the periphery of our Galaxy

Third Kepler's law

the second power of the orbital time grows proportionally to the third power of the orbit's semi-major axis (orbit's radius)

Second Kepler's law

areas swept by planets over equal intervals of time are equal

Orbital time of a comet

depends on the semi-major axis of its orbit

Orbit of Venus

an ellipse; the Sun is located in one of the two foci of this ellipse

The so-called 'Great Debate' of 1920

was a scientific discussion related to the nature and distance of spiral nebulae

Chemical elements in the Universe

dark energy (69%) and dark matter (26%)

Abundant chemical species in Jupiter and Saturn

hydrogen and helium

Abundant chemical elements in Earth's atmosphere

oxygen and nitrogen

Collision of Milky Way and Andromeda galaxies

formation of new stars would be enhanced within gaseous clouds of both galaxies

Milky Way Galaxy distance from Andromeda

separated by approximately 2.5 million AU from the Andromeda galaxy

Oort Cloud

a hypothetical reservoir of comets at the periphery of the Solar system

Kepler's law regarding orbital time

the ratio of the orbital time and of the orbit's semi-major axis is the same for all planets orbiting the same star

Kepler's law regarding force

the force acting on a body is equal to the product of the mass of this body and its acceleration

Orbital time of a comet

depends on the mass of a star it orbits

Orbit of Venus

a circle

Planets

non-stellar bodies orbiting a star, bodies massive enough to be rounded by gravity, and bodies massive enough to clear the neighborhood of their orbit of other bodies (except for moons that orbit around them)

Dwarf planets

non-stellar bodies orbiting the Sun; they are massive enough to be rounded by gravity but not massive enough to clear the neighborhood of their orbit of other bodies

Asteroids

rocky, metallic or icy bodies orbiting the Sun; they are smaller than planets and dwarf planets (except for Ceres that has a dual classification as a dwarf planet and an asteroid)

Comets

icy bodies orbiting the Sun; sizes of their icy cores are typically just a few miles across

Radiative energy in stars

released in cores of stars due to nuclear fusion reactions

Gravitational acceleration at the surface of Mars

is lower than the gravitational acceleration 100 miles above Mars' surface

Gravitational constant

is one of fundamental constants of nature and is needed for the calculation of gravitational force

Planetary nebula

an expanding gas nebula around a dying star; such a star will become a stellar remnant called a white dwarf

Range of stellar masses at birth

approximately 0.08 to 150 masses of the Sun

Hubble relation

galaxies recede from us at a speed that is proportional to their distance (the larger the distance, the higher the speed)

Globular clusters

very old and very massive star clusters; they typically contain hundred thousands to millions of stars

Open clusters

irregularly shaped star clusters, typically harboring hundreds to thousands of stars

Galactic year

the time the Sun needs to complete one orbit around the Galactic center

Morphological components of the Milky Way Galaxy

stellar disk with spiral arms, a rotating stellar bar in central parts of the Galaxy, a stellar halo made of individual stars and globular clusters, a stellar bulge

Trajectory of the Sun in our Galaxy

an ellipse

Halo of our Galaxy

contains dwarf galaxies, stellar streams made of disrupted dwarf galaxies, dark matter, individual stars and stars concentrated in globular clusters

Visible stars without a telescope

millions of stars in our Galaxy (all of its stars up to magnitude of about minus six)

Refracting telescopes

use a lens as a primary optical element

Blue light

has shorter wavelength than red light

X-rays

higher frequency than radio waves

Photons of visible light

higher energy than photons of infrared radiation

Typical sizes of nebulae and star clusters

100 l.y. for planetary nebulae, 1 l.y. for open clusters, 1000 l.y. for globular clusters, 100,000 l.y. for star forming nebulae

Distance estimation of the Andromeda galaxy

Henrietta Leavitt used Cepheid stars in this galaxy

One degree

contains 60 minutes of arc (60 arcminutes)

One radian

contains 57.3 degrees

1 parsec

approximately 3.26 light-years or 206,265 AU

1 light-year

approximately 63,241 AU

1 degree

approximately 200,000 seconds of arc

1 radian

approximately 200,000 seconds of arc

Stars in the Milky Way Galaxy

belong to a stellar disk with spiral arms, a rotating stellar bar, a stellar halo, and a stellar bulge

Galactic center

the time our Galaxy needs to complete one orbit around our Solar system

Galactic rotation

the time our Galaxy needs to make a full turn on its own axis

Dwarf galaxies in the halo

are part of the halo of our Galaxy

Dark matter in the halo

is part of the halo of our Galaxy

Angular size of the Moon

0.5 degrees

Angular size of the Moon

0.5 radians

Angular size of the Moon

1 minute of arc

Angular size of the Moon

1,800 seconds of arc

Astronomical unit

A unit of distance; it roughly corresponds to the average Sun-to-Earth distance

Astronomical unit

A unit of distance; such a distance would be visible under an angle of one second of arc from a distance of 1 parsec

Astronomical unit

A unit of time; it corresponds to the light travel time from Sun to Earth

Astronomical unit

A unit of distance; it is close to 150,000 kilometers

Parsec

A unit of distance; it corresponds to the distance traveled by light within approximately 3.26 years

Parsec

A unit of distance; it is equal to approximately 3.26 light years

Parsec

A unit of distance; it is equal to approximately 3.26 AU

Parsec

A unit of distance; it is defined as the distance from which the Sun-Earth separation is seen under an angle of 1 second of arc

Distance of the Earth from the center of our Galaxy

Approximately 27,000 times larger than the distance traveled by light in 1 year

Distance of the Earth from the center of our Galaxy

270,000 AU

Distance of the Earth from the center of our Galaxy

27,000 kiloparsecs

Distance of the Earth from the center of our Galaxy

150 million kilometers

Apparent stellar magnitude

A measure of locally measured stellar radiative flux

Large Magellanic Cloud (LMC)

Is visible by naked eyes from Earth's southern hemisphere

Large Magellanic Cloud (LMC)

Is a small spiral galaxy in the Local Group of galaxies

Large Magellanic Cloud (LMC)

Is a galaxy orbiting the Milky Way Galaxy

Small Magellanic Cloud

Is visible by naked eyes from Earth's southern hemisphere

Small Magellanic Cloud

Is a small galaxy in the Local Group of galaxies

Small Magellanic Cloud

Is a satellite galaxy of the Andromeda galaxy