Discovering the Universe for exam 1
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290 Terms
What is the cosmic perspective?
A way of understanding earth and humanity in the context of the enormous scales of space, time, and cosmic history.
How old is the solar system?
Approximately 4.6 billion years.
Approximately how much larger is the Sun than Earth in diameter?
About 100 times Earths diameter.
If the Sun were a grapefruit, how would Earth compare?
Earth would be approximately a pinhead 15 meters away.
How far away is the nearest star?
Proxima Centauri is about 4.2 light years away.
What is a solar system?
A star and all the material that orbits it.
What is a star?
A large, glowing ball of gas that produces energy through nuclear fusion.
What is a planet?
A moderately large, roughly spherical object that orbits a star and shines mainly through reflected light.
What is a moon?
An object that orbits a planet.
What is an asteroid?
A relatively small rocky object that orbits a star.
What is a comet?
A relatively small icy object that orbits a star.
What is a nebula?
An interstellar cloud of gas and dust observed through emission, reflection, or absorption.
What is a galaxy?
A large collection of stars and other matter held together by gravity and orbiting a common center.
Approximately how wide is the Milky Way?
About 100,000 light years.
Where is the Sun located in the Milky Way?
In a spiral arm about two-thirds of the way from the galactic center
What is the Local Group?
A group of more than 50 galaxies containing the Milky Way and Andromeda.
What is Laniakea?
The larger supercluster structure containing the Local Group.
What are galaxy clusters?
Groups of many galaxies bound together by gravity.
What are superclusters?
Enormous structures made up of groups or clusters of galaxies.
What is the filament structure of the universe?
The large-scale arrangement of galaxies into interconnected filaments separated by enormous low-density voids.
What are cosmic voids?
Large regions of space containing relatively few galaxies.
What is the speed of light?
Approximately 300,000 km/s (186,000 mi/s)
What is c?
The symbol used for the speed of light
Why is the speed of light important to astronomy?
Because light takes time to travel, so we observe distant objects as they existed in the past.
How far away is the Orion Nebula?
About 1,500 light-years
How far away is Andromeda
Approximately 2-2.5 million light-years
What is a light year
The distance light travels in one year; it is a unit of distance, not time.
Approximately how long is a light year?
About 10 trillion km or 6 trillion miles
what is an astronomical unit?
The average earth-sun distance, approximately 150 million km or 93 million miles
what is a light-second?
The distance light travels in one second, approximately 300,000 km
Why is there a limit to how far back we can see?
The universe has a finite age and light travels at a finite speed.
What is the observable universe?
The portion of the universe from which light has had enough time to reach us.
What was the Hubble Deep field?
A very long exposure of an extremely small, carefully chosen patch of sky that revealed thousands of distant galaxies
What wavelength does JWST primarily observe
Infrared
Why is infrared especially useful for studying the early universe
Light from very distant galaxies has been redshifted toward longer wavelengths as the universe expands.
What are population III stars?
Hypothesized first-generation stars formed primarily from hydrogen and helium, before earlier stars had enriched the universe with heavier elements
What elements dominated the early universe?
Mostly hydrogen and helium, with a tiny amount of lithium.
Where did heavier elements come from?
They were produced through nuclear fusion in stars and through energetic stellar events such as supernovae.
What is nuclear fusion?
The process of combining lighter atomic nuclei to form heavier nuclei, releasing energy.
What is the heaviest element normally produced by fusion in massive stellar cores?
Iron
How are elements heavier than iron produced?
They require energetic processes associated with stellar explosions and other extreme astrophysical events.
What is a supernova
A powerful explosion associated with the death of a massive star.
Why are supernovae important to later generations of stars and planets
They distribute heavy elements into space, enriching gas clouds that can form new stars and planets
What is the cosmic cycle of stellar material
Gas and dust form stars → stars create heavier elements → stars die and return material to space → enriched gas and dust form new stars and planets
How fast does earth rotate at the equator
About 1,670 km/h (1,000 mph)
How fast does Earth orbit the Sun
About 30 km/s or 108,000 km/h (67,000 mph)
What is the ecliptic plane?
The plane defined by earths orbit around the sun
What is earths axial tilt
About 23.5 degrees from a line perpendicular to the ecliptic plane
What star does Earths axis point approximately toward
Polaris, the north star
How long does the Sun take to orbit the Milky Way
Approximately 230 million years.
How long did New Horizons take to reach Pluto?
More than 9 years
When was Voyager 2 launched
1977
Approximately how fast does Voyager 2 travel
Roughly 50,000 km/h
Approximately how long would Voyager 2 take to reach Alpha Centauri at its current speed if it were traveling in that direction
Roughly 100,000 years
What is the Hubble-Lemaître Law?
The relationship stating that a galaxys recession speed is generally proportional to its distance from us
What is the Hubble flow?
The large-scale recession of galaxies caused by the expansion of space.
What is redshift?
The shifting of light toward longer wavelengths, which for distant galaxies is evidence of cosmic expansion
How do astronomers measure galaxy motion
By examining the spectra of their light and measuring Doppler/redshift effects
What is dark matter?
Matter that does not emit enough detectable light to be directly observed but is inferred from its gravitational effects
How do we know dark matter exists?
The motions of stars and galaxies indicate that there is much more mass present than the visible matter can account for.
Where is much of the Milky Ways dark matter thought to be
In a large halo surrounding the visible disk
What is dark energy
The name given to whatever is responsible for the observed accelerated expansion of the universe.
What is the difference between dark matter and dark energy
Dark matter is inferred primarily from gravity and affects the motion/structure of matter; dark energy is associated with the acceleration of cosmic expansion
What is the celestial sphere?
An imaginary sphere surrounding Earth that astronomers use as a convenient mental map for locating objects in the sky.
Are stars actually attached to the celestial sphere?
No. They are at different distances from Earth and only appear to lie on the same sphere because of our lack of depth perception
What is a constellation?
A formally defined region of the sky with official boundaries.
Do the stars in a constellation necessarily have a physical relationship with each other?
No. They may be at very different distances from Earth.
What is an asterism?
A recognizable pattern of stars that is not necessarily an official constellation.
Give an example of an asterism.
The Big Dipper, which is part of Ursa Major.
Why do constellations appear flat?
Because we lack depth perception when looking across enormous astronomical distances.
What is the north celestial pole?
The point on the celestial sphere directly above Earth's North Pole.
What is the south celestial pole?
The point on the celestial sphere directly above Earth's South Pole.
What is the celestial equator?
Earth's equator projected outward onto the celestial sphere.
Why are the celestial equator and ecliptic tilted by 23.5°?
Because Earth's rotational axis is tilted 23.5° from perpendicular to the ecliptic plane.
What is the horizon?
The boundary between Earth and sky from an observer's location.
What is the zenith?
The point directly overhead.
What is the meridian?
An imaginary half-circle running from due south on the horizon, through the zenith, to due north.
What is altitude?
The angle of an object above the horizon.
What is azimuth?
The direction of an object along the horizon, measured clockwise from due north.
What are right ascension and declination?
A coordinate system used to locate objects on the celestial sphere, similar to longitude and latitude on Earth.
What does latitude measure?
North-south position on Earth.
What does longitude measure?
East-west position on Earth.
Where is 0° longitude?
Along the prime meridian through Greenwich, England.
What is the relationship between latitude and the altitude of the celestial pole?
The altitude of the celestial pole equals your latitude.
What are circumpolar stars?
Stars that never go below the horizon and therefore never set
What happens to the sky as Earth rotates?
It appears to rotate in the opposite direction around the celestial poles.
What would the sky look like from the North Pole?
Stars circle around the north celestial pole without rising or setting, and you only see the northern celestial hemisphere.
What happens at the equator?
Stars rise and set nearly perpendicular to the horizon, and over time essentially the entire celestial sphere can be seen.
What causes Earth's seasons?
Earth's 23.5° axial tilt, not its changing distance from the Sun.
Why is summer warmer?
The hemisphere is tilted toward the Sun, producing more direct sunlight and longer days.
Why is winter colder?
The hemisphere is tilted away from the Sun, producing less direct sunlight and shorter days.
What is the June solstice?
The point when the Northern Hemisphere is tilted most toward the Sun; around June 21, it is the Northern Hemisphere's summer solstice and longest day.
What is the December solstice?
The point when the Northern Hemisphere is tilted most away from the Sun; around December 21, it is the Northern Hemisphere's winter solstice and shortest day.
What is the March equinox?
Around March 21, when neither hemisphere is tilted toward or away from the Sun and day/night are approximately equal.
What is the September equinox?
Around September 22, when neither hemisphere is tilted toward or away from the Sun and day/night are approximately equal.
What is the easiest distinction between solstices and equinoxes?
Solstices = maximum tilt toward/away; equinoxes = neither hemisphere tilted toward/away.
What is precession?
The slow wobbling/change in orientation of Earth's rotational axis.
How long does Earth's precession cycle take?
About 26,000 years.
Why does Earth's axis precess?
Earth's equatorial bulge interacts gravitationally with the Sun and Moon, producing the slow wobble.
What effect does precession have on the North Star?
The identity of the star nearest the north celestial pole changes over thousands of years.