FInal!!! Flashcards
How does our galaxy look like to us and to the outside observer?
Within: bright, star-filled band in the night sky
Outside: well-defined arms, central bulge, surrounding halo
Main stellar components of the Milky Way
- Disk and spiral arms
- The bulge
- Stellar halo
How the stars orbit within our galaxy(and other spiral galaxies)
Stars in our galaxy, and other spiral galaxies, orbit the galactic center in nearly circular paths within the galactic plane.
The orbital speed of stars generally remains constant regardless of their distance from the center, leading to a phenomenon known as “flat rotation curves.”
How the orbital patterns differ for the various components? (disk, bulge, and stellar halo).
Disk: Follow nearly circular orbits around the galactic center. These orbits are relatively orderly and lie close to the galactic plane.
Bulge: Stars in the central bulge have more random and elliptical orbits, moving in a more chaotic manner. These orbits are not confined to the plane of the galaxy and can be highly inclined.
Stellar halo: Stars in the stellar halo orbit the galaxy in highly elliptical and irregular paths. These orbits are the most chaotic and extend far beyond the main structure of the galaxy, often in any direction.
What does the “rotation curve” of the Milky Way and other spiral galaxies looks like and how it provides evidence for dark matter in galaxies?
The “rotation curve” of the Milky Way and other spiral galaxies plots the orbital speeds of stars and gas against their distance from the galactic center. Contrary to expectations from visible matter alone, these curves remain flat or even rise at greater distances, rather than declining. This discrepancy suggests the presence of dark matter, providing the extra gravitational pull needed to maintain these high orbital speeds far from the galactic center.
Evidence for a supermassive black hole in the center of our galaxy
Orbits of stars in the Galactic Center, and high-resolution images from the Event Horizon Telescope
The 3 main types of galaxies, their overall structure, and how they differ. - Spiral (or disk) galaxies, elliptical galaxies, irregular galaxies
Spiral (or Disk) Galaxies:
~Structure: Characterised by a flat, rotating disk containing stars, gas, and dust, along with a central bulge and spiral arms. The disk is surrounded by a halo of older stars and globular clusters.
~Examples: Milky Way, Andromeda.
~Differences: Have well-defined structures with spiral arms where active star formation occurs. They often appear as a combination of blue (young stars) and yellowish (older stars) colours.
Elliptical Galaxies:
~Structure: Range from nearly spherical to elongated, with a smooth, featureless appearance. They lack the spiral structure and have little gas and dust, meaning minimal star formation.
~Examples: M87, NGC 5128.
~Differences: Composed mostly of older, red stars and appear more uniform in colour. They can vary greatly in size from dwarf ellipticals to giant ellipticals.
Irregular Galaxies:
~Structure: Do not have a distinct shape like spirals or ellipticals. They often appear chaotic, with no central bulge or spiral arms.
~Examples: Large Magellanic Cloud, Small Magellanic Cloud.
~Differences: Contain abundant gas and dust, leading to active star formation. They often look patchy or fragmented, and their appearance can be influenced by gravitational interactions with other galaxies.
What is the “distance ladder”?
The “distance ladder” is a series of methods by which astronomers determine the distances to celestial objects.
Includes:
1. Parallax: Used for the nearest stars, this method measures the apparent shift in a star’s position as seen from Earth at different points in its orbit.
- Cepheid Variables: These are pulsating stars with a well-defined relationship between their luminosity and pulsation period, allowing their true brightness to be determined and distances to be calculated.
- Type Ia Supernovae: These supernovae have a consistent peak brightness, serving as “standard candles” to measure distances to faraway galaxies.
- Redshift: For the most distant objects, the redshift of their light due to the expansion of the universe provides a measure of their distance.
how Edwin Hubble proved that other galaxies lie far beyond the Milky Way?
- Cepheid Variables
- Period-Luminosity Relationship: By observing the period of a Cepheid’s pulsation, Hubble could determine its true brightness.
- Apparent Brightness: Hubble measured the apparent brightness of Cepheid variables in the Andromeda Galaxy.
- Distance Calculation: Using the inverse-square law of light, which relates the true brightness and apparent brightness to distance, Hubble calculated the distances to these Cepheids.
- Beyond the Milky Way: The distances he found were much greater than the size of the Milky Way, proving that Andromeda and other galaxies were separate “island universes” far beyond our own galaxy.
What is Hubble’s Law?
- The distance ladder consists of interconnected “rungs”: radar (in the Solar System), parallax, and different types of standard candles (including Cepheid variable stars and white dwarf supernovae)
- Hubble’s Law: a galaxy’s distance is proportional to its recessional velocity, v = H0 × D.
- Hubble’s Constant H0 describes the current expansion rate of the Universe.
- Remember how we measure velocities of galaxies towards/away from us, using the Doppler shift: v c = ∆λ λ0
How Hubble’s Law tells us the approximate age of the Universe?
We can rewrite Hubble’s Law in the form of Distance = Speed × Time, and estimate the amount of time as the age of the Universe. This approach assumes the expansion rate has been constant, which is not true, so it’s only a rough approximation.
How we can observe the life histories of galaxies, by looking at galaxies at different distances (= different “look-back times”)?
Speed of Light: Light from distant galaxies takes time to reach us, so when we observe these galaxies, we see them as they were in the past.
Look-Back Time: The further away a galaxy is, the longer its light has traveled, allowing us to see it at an earlier stage in its development.
Chronological Snapshots: By observing galaxies at various distances, we get snapshots of different stages in their evolution, from early formation to mature systems.
Comparative Study: By comparing these snapshots, astronomers can piece together the typical life cycles of galaxies, understanding how they form, evolve, interact, and sometimes merge over billions of years.
The basic process of galaxy formation, according to our most successful models
Initial Density Fluctuations: Small fluctuations in the density of matter in the early universe, amplified by gravitational instability, served as the seeds for galaxy formation.
Dark Matter Halo Formation: Dark matter, which does not interact with light, began to clump together due to gravitational attraction, forming the initial framework or “halos” within which galaxies could form.
Gas Cooling and Condensation: Gas within these dark matter halos cooled and condensed, falling into the center to form stars and gas clouds, leading to the formation of the first protogalaxies.
Star Formation: As gas continued to cool and condense, it formed stars, star clusters, and eventually organised into larger structures such as disk galaxies.
Mergers and Interactions: Galaxies grew and evolved through mergers and interactions with other galaxies, which could trigger bursts of star formation and lead to the development of larger and more complex structures.
How gas cycles within and in/out of galaxies?
-Gas from dying stars returns to the interstellar medium and provides material for new stars to form.
-Supernovae and other energy released from stars can drive galactic winds, ejecting gas from a galaxy (especially in starburst galaxies).
-Gas which stars return to the interstellar medium contains the new heavy elements created by those stars, such that the total heavy element content of gas and newly-formed stars tends to increase over time.
How we can observe the gas outside of galaxies?
This gas can be detected via absorption lines in the spectra of distant bright objects.
