# Stars/Blues

Stars are often referred to by their predominant color. For example, blue stragglers are found among the galactic halo globular clusters. Blue main sequence stars, that are metal poor, ([Fe/H] ≤ -1.0) are most likely not analogous to blue stragglers.

## Blue halo stars

"The distribution of the high-latitude faint blue stars over T$_{eff}$ ... [shows] that the principal sequence [has] two gaps, at colors corresponding to log T$_{eff}$ ~ 4.11 (gap 1) and log T$_{eff}$ ~ 4.33 (gap 2). ... [T]he gaps [may be] a horizontal-branch phenomenon. ... [C]urrent theoretical concepts of the advanced evolution of Population II stars can explain the majority of blue halo stars.”

## Blue stragglers

“Blue stragglers (BSS) are main sequence stars in open or globular clusters that are more luminous and bluer than stars at the main sequence turn-off point for the cluster. ... Standard theories of stellar evolution hold that the position of a star on the Hertzsprung–Russell diagram should be determined almost entirely by the initial mass of the star and its age. In a cluster, stars all formed at approximately the same time, and thus in an H–R diagram for a cluster, all stars should lie along a clearly defined curve set by the age of the cluster, with the positions of individual stars on that curve determined solely by their initial mass. With masses two to three times that of the rest of the main sequence cluster stars, blue stragglers seem to be exceptions to this rule. The resolution of this problem is likely related to interactions between two or more stars in the dense confines of the clusters in which blue stragglers are found.”

## Hypervelocity stars

"To date, all of the reported hypervelocity stars (HVSs), which are believed to be ejected from the Galactic center, are blue and therefore almost certainly young.”

## Luminous blue variables

"Luminous blue variables, also known as S Doradus variables, are very bright, blue, hypergiant variable stars named after S Doradus, the brightest star of the Large Magellanic Cloud. They exhibit long, slow changes in brightness, punctuated by occasional outbursts in brightness during substantial mass loss events (e.g. Eta Carinae, P Cygni). They are extraordinarily rare. The General Catalogue of Variable Stars only lists 20 objects as SDor."
"LBVs can shine millions of times brighter than the Sun and, with masses up to 150 times that of the Sun, approaching the theoretical upper limit for stellar mass, making them among the most luminous, hottest, and most energy-releasing stars in the universe. If they were any larger, their gravity would be insufficient to balance their radiation pressure and they would blow away the excess mass through stellar wind. As they are, they barely maintain hydrostatic equilibrium because their stellar wind constantly ejects matter, decreasing the mass of the star. For this reason, there are usually nebulae around such stars created by these outbursts; Eta Carinae is the nearest and best-studied example. Because of their large mass and high luminosity, their lifetime is very short — only a few million years."

## Blue outliers

"There are faraway active galaxies that show a blueshift in their emission lines. One of the largest blueshifts is found in the narrow-line quasar, PG 1543+489, which has a relative velocity of -1150 km/s. These types of galaxies are called "blue outliers".”

## Blue dwarfs

"The Algol-type binaries are close, semi-detached, interacting binary star systems which contain a cool F–K giant or sub-giant secondary star that fills its Roche lobe and is losing mass to a hot B–A main sequence primary star."
"The Algols contain a hot blue dwarf star with a magnetically-active late-type companion. In the close Algols, the gas stream flows directly into the photosphere of the blue mass-gaining star because it does not have enough room to avoid impact with that star."

## Blue giants

“[A] blue giant is a star with a spectral type of O or B (thus being noticeably blue in appearance) and a luminosity class of III (giant). In the standard Hertzsprung-Russell diagram, blue giants are found in the upper left corner, due to their high luminosity and early spectral type.”
Def. "a very hot and very luminous star that emits visible light in the blue portion of the spectrum" is called a blue giant.

## Blue-horizontal-branch stars

Notation: let the symbol BHB stand for Blue horizontal-branch.
"BHB stars are excellent tracers of Galactic halo dynamics because they are luminous and have a nearly constant absolute magnitude within a restricted color range"

## Blue subdwarfs

Blue “subdward B [stars] ... are difficult to fit into the evolutionary scheme.”
"Subdwarf B stars were discovered by Zwicky and Humason around 1947 when they found subluminous blue stars around the north galactic pole. In the Palomar-Green survey they were discovered to be the commonest kind of faint blue star with a magnitude over 18. During the 1960s spectroscopy discovered that many of the sdB stars are deficient in helium, with abundances below that predicted by the big bang theory. In the early 1970s Greenstein and Sargent measured temperatures and gravity strengths and were able to plot their correct position on the Hertzsprung–Russell diagram."

## Blue supergiants

“Blue supergiants (BSGs) are supergiant stars (luminosity class I) of spectral type O or B. They are extremely hot and bright, with surface temperatures of 30,000-50,000 K. They ... can have radii up to about 25 solar radii. These rare and enigmatic stars are amongst the hottest and brightest in the known Universe.”
Def. "a very large, hot and luminous star; a large blue giant" is called a blue supergiant.

## Galaxies

The galaxy, called NGC 5010, is in a period of transition. The aging galaxy is moving on from life as a spiral galaxy, like our Milky Way, to an older, less defined type called an elliptical galaxy. In this in-between phase, astronomers refer to NGC 5010 as a lenticular galaxy, which has features of both spirals and ellipticals.
"Schweizer [1978] defined five observational characteristics that unambiguously identified NGC 7252 as a late-stage merger ... One of these new characteristics is the blue portion of the optical spectrum of the main body."