Variable Star
[ The expansion of the light echo of a red variable star, the V838 Monocerotis. Credit: HST ACS/WFC ]
After the identification of the first variable star in 1638 by Johannes Holwarda (Mira variable originally designated as Omicron Ceti), the conventional belief of never-ending immutability of a starry sky turned out to be wrong: "The Heavens are Changing". Since that time, monitoring brightness changes of various sources in the sky has provided crucial informations for astronomers for centuries.
For instance, Lemaître (1927) and Hubble (1929) found the first evidence of the expanding universe using a relation between distance and radial velocity of galaxies, as shown in Figure 1. The distance was derived using a Period-Luminocity relation (P-L relation) of Cepheid variables.
The P-L relation also gave a resolution to the Great Debate between H. Shapley and H. Curtis (Trimble 1995) that is a still interesting reading even today showing how incomplete and partly proven observational evidences could lead to incorrect speculations.
[ Figure 1. The evidence of the expanding universe. The figure shows a relation between distance (x-axis) and radial velocity (y-axis) of galaxies. “The black discs and full line: the solution for solar motion using the galaxy individually. The circles and broken line: the solution combining the galaxy into groups. The cross: the mean velocity corresponding to the mean distance of 22 galaxies whose distances could not be estimated” (Hubble, 1929). ]
In 1968, A. Hewish and S. J. Bell discovered the first pulsar by detecting rapidly pulsating signals using radio observations (Hewish+ 1968). He was the first astronomer awarded the first Novel Prize because of the importance of this discovery resulting in many important informations in stellar physics such as 1) the first detection of a previously unknown type of a star: a neutron star, 2) an evidence of gravitational radiation predicted by general relativity (Taylor & Weisberg 1982), and 3) the first confirmed extrasolar planet orbiting around a pulsar PSR 1257+12 (Wolszczan & Frail 1992).
[ Composite Optical/X-ray image of the Crab Nebula, having a pulsar at its center. Credit: NASA ]
An another astonishing discovery in 20th century was a detection of an observational evidence for accelerated expansion of the universe using light curves of Type Ia supernova explosions (Riess+ 1998). Not surprisingly, this groundbreaking discovery awarded him and his colleagues the Nobel Prize.
[ Supernova Type I progenitor. Credit: NASA ]
Now, why don't we start a journey to this exciting world?