Class 4. Universe
Recommended Article : 【Earth Science】 Table of Contents for Earth Science
1. Universe
3. Nebulae
4. Galaxies
5. Cosmic Structures of the Universe
b. Calculating the Time for Stars to Form in Nebulae
1. Universe
⑴ Characteristics of the Universe
① Age : 13.798 billion years
② Average Color of the Universe : Cosmic Latte. Assigned by astronomers at Johns Hopkins University
⑵ Evolution of the Cosmos
⑶ Habitable Zone
① Habitable Zone
Figure. 1. Habitable Zone
② Drake Equation : Equation for the number of communicative extraterrestrial civilizations
N = R* × fp × ne × fl × fi × fc × L
○ N : Number of communicative extraterrestrial civilizations
○ R* : Number of stars formed per year
○ fp : Fraction of stars with planets
○ ne : Number of planets in the habitable zone per star with planets
○ fl : Fraction of planets in the habitable zone where life arises
○ fi : Fraction of planets with life that evolves to intelligent life
○ fc : Fraction of intelligent civilizations that can communicate across planets
○ L : Lifespan of civilizations
2. Star Clusters
⑴ Definition : A group of stars that are densely packed within a region of space
⑵ Classification 1. Open Clusters
① Definition : Star clusters where stars are young and dispersed
② Number of Stars : Tens to thousands ( ∵ Stars are young)
③ Color : Blue ( ∵ High temperature due to youth)
④ Distribution Area : Spiral Arms ( ∵ Recently captured by the galaxy)
⑤ Main Feature : Main sequence stars are prominent on the H-R diagram
⑶ Classification 2. Globular Clusters
① Definition : Star clusters where stars are old and arranged in a spherical shape
② Number of Stars : Hundreds of thousands to millions ( ∵ Stars are old)
③ Color : Red ( ∵ Low temperature due to age)
④ Distribution Area : Halo, Central Bulge ( ∵ Central movement over time)
⑤ Main Feature : Red giant branch and horizontal branch exist on the H-R diagram
3. Nebulae
⑴ Definition : Celestial bodies that have a cloud-like appearance
⑵ Interstellar Dust
① Interstellar Dimming : Dimming of light due to interstellar dust, leading to higher apparent magnitude
② Interstellar Reddening : Red light scatters less, making it easier to observe
○ Excess Color = Observed (B - V) - Intrinsic (B - V)
③ Interstellar Blueing : When interstellar matter scatters starlight, it is mainly observed as blue
⑶ Interstellar Gas
① Region 1. Molecular Clouds : Hydrogen molecules. Lower temperature, higher density. Birthplace of stars
② Region 2. HⅠ Region : Neutral hydrogen. Emits 21 cm radio waves → Discovery of spiral arms in our galaxy
③ Region 3. HⅡ Region : Ionized hydrogen. Higher temperature, lower density
⑷ Types of Nebulae
① Dark Nebulae (e.g., Horsehead Nebula)
② Reflection Nebulae (e.g., Pleiades Nebula)
○ Interstellar dimming, interstellar reddening, interstellar blueing occur
③ Emission Nebulae (or Extragalactic Nebulae) (e.g., Orion Nebula)
○ O and B-type stars present within emission nebulae
○ 1st. Emission of ultraviolet radiation by stars
○ 2nd. Surrounding hydrogen ionization
○ 3rd. Ionized hydrogen combines with surrounding electrons, returning to neutral state
○ 4th. Emission of red visible light
⑸ Extragalactic Nebulae beyond Galaxy : Observed by the Hubble Law
4. Galaxies
⑴ Types of Galaxies
① Normal Spiral Galaxy
② Barred Spiral Galaxy
○ Contains a bar-shaped structure crossing the nucleus
③ Elliptical Galaxy
○ Low interstellar matter. Classified from E0 to E7 based on flattening
○ Higher proportion of older stars, like in globular clusters
④ Irregular Galaxy
○ Higher proportion of younger stars, like in open clusters
⑵ Example 1. Milky Way
Figure. 2. Profile of the Galaxy 1
① Diameter : 98,000 light-years
② Shape : Barred spiral galaxy
③ Central Region : Primarily open clusters
④ Outer Region : Primarily globular clusters
⑤ Halo : A spherical region surrounding the galactic disk
○ Mass of the halo is comparable to the total mass of luminous objects in the galaxy
⑥ Research that Revealed the Milky Way’s Spiral Structure
○ Energy : Proton and electron spinning in the same direction (gyration) > Proton and electron spinning in opposite directions
○ Difference in energy levels corresponds to the energy of 21 cm hydrogen line
○ Due to the rotation of the Milky Way, the Doppler effect causes the 21 cm hydrogen line emitted by neutral hydrogen
○ Line-of-sight velocity maps intensity → Amount of hydrogen, hydrogen’s location → Proves the existence of spiral arms
⑶ Example 2. Andromeda Galaxy
① Among 20 galaxies around the Milky Way, only the Andromeda Galaxy is larger
② Initially classified as a star, later identified as a galaxy due to advances in observation technology
⑷ Quasars
① Enormous galaxies appearing like stars due to their extreme distance
② Shows a similar apparent magnitude as
stars in the Milky Way
③ Exhibits strong redshift, indicating it’s a very distant extragalactic object
5. Cosmic Structures of the Universe
⑴ Galaxies : Star Clusters + Nebulae
⑵ Galaxy Groups
① Dozens of galaxies
② 1-2 Mpc
③ Gravitationally bound O
④ e.g., Milky Way (Spiral), Andromeda Galaxy (Spiral), Large and Small Magellanic Clouds
⑶ Galaxy Clusters
① Hundreds to thousands of galaxies
② 2-10 Mpc
③ Gravitationally bound O
④ e.g., Virgo Cluster (Primarily spiral, central ellipticals)
⑷ Superclusters
① Aggregations of galaxy groups and clusters
② Tens to hundreds of Mpc
③ Not gravitationally bound X
④ e.g., Virgo Supercluster
⑸ Cosmic Structures
① Imaginary structures of the universe formed by density variations in the early universe + dark matter
② Structure 1. Galaxy Superclusters : High density, galaxies present, dark matter present
③ Structure 2. Cosmic Voids : Low density, no galaxies, dark matter present
Input : 2019.04.07 10:27