Neptune

The T Tauri stars (classified as INT) vary irregularly in brightness, whereas the FU Orionis stars (FU) exhibit a single brightening, possibly as they assume a stable state. Nebular variables are F-, G- or K-type giant stars, many still surrounded by warm cocoons of dust and gas. There are a great number in the orion nebula. While contracting, nebular variables probably lose large amounts of material in the form of a stellar or T-Tauri wind. The rate of mass...

Hertzsprung Russell diagram

Began to establish the mathematical form of Henrietta Leavitt's period-luminosity relation for Cepheid variable stars, and used it to determine accurately the distance to the Small Magellanic Cloud. Hertzsprung's work spanned the extremes of stellar types the Cepheids are highly luminous yellow supergiants and, in contrast, he was the first to classify, in 1911, the giant and dwarf subdivisions of late-type stars. The culmination of this work, based in part on the work of pioneering astronomer...

Saturn Nebula

The final Saturn IB launch took place in 1975 during the Apollo-Soyuz Test Programme. saros Cycle of 18 years 10.3 days after which the Sun, Moon and the nodes of the Moon's orbit return to almost the same relative positions. It is a result of the regression of the nodes. The saros was known to the ancient Babylonian astronomers it was used to predict eclipses, since any eclipse is usually followed by a similar one 18 years 10.3 days later. Slight variations from cycle to cycle mean that the...

T

Tully-Fisher relation Relationship between the widths of the 21-cm radio emission line of hydrogen (see twenty-one centimetre line) emitted by giant gas clouds in spiral galaxies to the optical brightnesses of the galaxies. The larger the galaxy, the faster the rotation and the broader the emission line width, and also the brighter the galaxy the more stars it has. This relationship can be used to measure distances to galaxies out to about 100 Mpc. Tunguska event Very bright fireball that was...

Morgan Keenan classification

Morgan, William Wilson (1906-94) American astronomer whose work led to the Morgan-Keenan system of spectral classification and the UBV system of photometry. He spent his entire career at Yerkes Observatory (1926-94), briefly serving as its director (1960-63). Assisted by Philip Childs Keenan (1908-2000) and Edith Marie Kellman (1911- ), Morgan classified huge numbers of stars into six different luminosity classes. This luminosity classification was combined with a new two-dimensional spectral...

Tiltedcomponent telescope TCT reflecting

Telescope that avoids obstructing the optical path by tilting or warping some or all of the optical elements. The first attempt at a tilted-element telescope was the brachyt, developed around 1900. This was essentially a cassegrain telescope cut in half, and it had off-axis mirrors that were virtually impossible to grind. An improvement was the neo-brachyt in 1953 it had normal mirrors but required a warping harness to distort the secondary into the proper curve. The schiefspiegler, invented in...