Astro Lecture 1 Flashcards
What did early naked eye observations reveal about celestial bodies?
Many ‘fixed’ stars (maintaining relative positions) and seven moving celestial bodies: Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn
What was the Greek geocentric model of the universe?
Eight rotating spheres around Earth: seven for the Sun, Moon, and planets, and one for the stars on the outside
How did Ptolemy improve upon earlier geocentric models?
By adding epicycles to explain irregular planetary motions, eventually requiring approximately 50 spheres rather than the original 8
Who created the first known heliocentric model and when?
Aristarchus of Samos, around 270 BC
What is Olber’s paradox?
The question of why, in an infinite universe, the night sky is dark instead of uniformly bright
What important discoveries did Galileo make with his telescope in the 1610s?
Moons around Jupiter, phases of Venus, and sunspots (indicating the Sun’s rotation)
What theoretical advance did Kepler make in 1609?
He introduced elliptical orbits, replacing the need for epicycles in heliocentric models
What evidence of Earth’s motion did James Bradley discover in 1725?
Stellar aberration, which is the apparent shift in star positions due to Earth’s motion through space
What is stellar parallax and why is it significant?
The apparent shift in a star’s position when viewed from different points in Earth’s orbit; it provides direct evidence that Earth orbits the Sun
What equation represents Einstein’s General Relativity?
Gμν = 8πTμν
Why did Einstein add the cosmological constant to his equations?
To balance gravitational attraction and maintain a static universe model
Why did Einstein later call the cosmological constant his ‘biggest blunder’?
Because the discovery of the expanding universe made it unnecessary
What was the ‘Great Debate’ of 1920 about?
Whether nebulae were clouds of dust within the Milky Way (Shapley) or distant galaxies like the Milky Way (Curtis)
How did Edwin Hubble resolve the ‘Great Debate’?
By identifying individual stars in Andromeda and establishing its distance at ~1 million light years, proving nebulae were other galaxies
What two key findings led to the discovery of the expanding universe?
1) Everything is moving away from us
2) Objects further away move faster
What is the correct sequence in our cosmic address from smallest to largest?
Earth → Solar System → Milky Way galaxy → Local Group of galaxies → Laniakea Supercluster → Cosmic Web
What is the Local Group?
A collection of more than 54 galaxies, including the Andromeda Galaxy and our Milky Way
What is Laniakea?
Our supercluster containing approximately 100,000 galaxies with a diameter of about 520 million light-years
What is the Cosmic Web?
The large-scale structure of the universe, showing galaxies arranged in filaments and clusters with vast voids between them
What is the approximate diameter of the Milky Way galaxy?
5×10²⁰ m (about 100,000 light-years)
How far is the nearest star (Alpha Centauri) from our Solar System?
4×10¹⁶ m (about 4.3 light-years)
What is the radius of Earth?
6.4×10⁶ m (6,400 km)
