Stellar Classification and Properties Quiz

Observational Facts

• Observational astronomy has advanced significantly since the 1940s, expanding beyond optical wavelengths to encompass the entire electromagnetic spectrum, aided by advancements in computer technology and the increasing number of astronomers.
• Data acquisition is vast, making a comprehensive overview impossible within a single chapter.
• Astronomical observations rely primarily on the radiation emitted or absorbed by celestial objects.
• Earth's atmosphere limits observations to certain wavelengths, requiring space-based detectors for some parts of the electromagnetic spectrum (e.g., far-infrared, ultraviolet, X-ray, gamma-ray).
• The energy density spectrum displays the distribution of cosmic background radiation across different wavelengths, with the cosmic microwave background (CMB) dominating lower energies.
• Galactic starlight, dust emission, and hot gas in galaxy clusters are dominant radiation sources at various parts of the spectrum.

Astronomical Observations

• The spectral energy distribution (SED) of a source describes the energy emitted at different wavelengths, from radio to gamma rays.
• Optical, near-infrared, and radio wavelengths have relatively clear atmospheric transmission windows.
• Flux is the total energy received per unit area per unit time from a unit solid angle in a specific direction.
• Bolometric luminosity is the total energy output from an object across all wavelengths.
• Astronomical objects' size is usually characterized by isophotal contours or radii that enclose a certain fraction of the total flux (like the half-light radius).
• Magnitude is a logarithmic measure of flux, often standardized to a reference star.
• Color indices are the difference in magnitudes between different bands, providing information about the overall spectral shape.

Fluxes and Magnitudes

• Integrated surface brightness estimates total flux or luminosity.
• Distance modulus is the difference between apparent and absolute magnitudes and is used to estimate distances.
• Surface brightness expressed in magnitudes/square arcsecond is related to physical units

Spectroscopy

• Spectroscopy provides detailed information regarding the radiation spectrum, enabling extraction of valuable insights about an objects' physical conditions (temperature, ionization, and composition).
• Spectral analysis reveals a continuum component (usually from the combined light of stars in the galaxy) overlaid with emission and/or absorption lines.
• These lines emanate from atoms within the galaxy's atoms, ions, and molecules, thereby offering insights into gas and dust.
• Line broadening in a spectrum can reveal information about stellar and/or gas motions.
• Redshifts in a spectrum provide relative speed/distance.

Distance Measurements

• Trigonometric parallax is a method employed for calculating distances to nearby stars by observing their apparent position shift against a background of distant objects over a fixed period (such as six months), utilizing the Earth's orbit as a baseline.
• Motion-based methods determine distances using a star's proper motion (angular displacement per unit time) against the background, combined with its radial velocity

Standard Candles and Standard Rulers

• Standard candles have known absolute luminosities, enabling distance estimations for more distant objects (e.g., Cepheid variable stars and Type Ia supernovae).
• Standard rulers have known physical sizes and can determine distances by using their apparent angular sizes.

Stars

• The Hertzsprung-Russell (H-R) diagram displays the correlations between luminosity and temperature, aiding in the study of stellar evolutionary stages and properties.

Galaxies

• Galaxies are typically categorized using the Hubble sequence, a morphological classification system that categorizes galaxies into ellipticals, lenticulars, spirals, and irregulars based on their shapes and structures.
• The morphology-density relation indicates a correlation between galaxy morphology and the spatial density of galaxies within clusters.
• Dwarf galaxies are a class of galaxies with lower luminosities than typical galaxies, and their properties vary widely across different types.
• Surface brightness profiles of elliptical galaxies follow the Sérsic profile (R1/n), characterized by a central brightness and an effective radius.
• The luminosity, sizes, and colors of galaxies in different environments (like clusters) vary systematically
• Disk galaxies, comprised of a thin disk and bulge region, also exhibit variations in structure and properties.
• The properties of a galaxy (like its structure and the relative brightness of its components) varies considerably from galaxy to galaxy.
• Active galactic nuclei (AGN) are exceptionally bright regions located at the center of some galaxies.
• The evolution of the galaxy population can be tracked through studies of high redshift and faint galaxies
• The star-formation history of the universe is mapped by observation of star-forming galaxies.

The Cosmic Microwave Background

• The CMB is a faint afterglow of the Big Bang, detectable at microwave wavelengths, providing information about the early universe.
• CMB observations reveal that the universe is homogeneous and isotropic (with only small deviations), consistent with the cosmological principle.

The Intergalactic Medium

• The intergalactic medium (IGM), the matter between galaxies, is mostly neutral/ionized gas.
• The presence and density of neutral hydrogen in the IGM can be investigated by observations of absorption lines in quasar spectra.
• Absorption features in the quasar spectra related to the absorption of neutral hydrogen help to study the history of reionization.

Large-Scale Structure

• The spatial distribution of galaxies is inhomogeneous, displaying clumps and filaments, voids.
• Two-point correlation functions quantify the clustering of galaxies, measuring excess galaxy pairs of a given separation, which is a useful parameter for assessing the clustering of matter.

Observational Details

• Observational techniques involve various measures for quantities of the CMB (like its temperature fluctuations, angular power spectrum) and galaxies (e.g. their surface brightness profiles and redshift).
• Some of the difficulties in interpreting data include the different wavelengths at which data are measured, cosmological evolution that can significantly affect estimates, and also the need to correct for the systematic effects (like cosmological redshift) of the objects under consideration