The Whole World In Your Hand

Could you understand the stars and planets by using a devise small enough to fit on your desk? Before telescope, globes and modern planetariums, astronomers and mathematicians wanted tools to help them figure out celestial objects’ locations and predict their motions. Luckily, the ancient Greek invented the armillary sphere, an object that demonstrated the motions of celestial objects through the sky. (This predates the idea that the Earth and planets move around the Sun. Later, the design of these spheres changed so that the Sun-and not the Earth- was positioned in the center. People still own armillary sphere today, but they are mostly used for decoration.

Globular Clusters

Dotted around our Galaxy in a spherical halo are more than a hundred ball shaped conglomerations of stars known as Globular Clusters. These each contain from a hundred thousand to several million stars, all bound together by gravity. Several globular clusters are visible with the naked eye or binoculars. The brightest are ω (omega) Centauri and 47 Tucanae, both is in southern hemisphere the best example is M13(picture above) in Hercules.
To the naked eye and in binoculars, these objects appears as softly glowing patches of light. Moderate size telescopes stars to resolve some of the individual red giant stars, giving the clusters a speckled appearance.
Globular Clusters formed early in the history of the galaxy and contains some of the most ancient stars known, 10,000 million years and more old, or over twice the age of our Sun.

Black Hole

If the core of exploded star has a mass of more than three Suns, then even a neutron star is not the end for it. Instead, its become something more bizarre: a black hole. No force can shore up a dead star weighing more than tree solar masses against the inward pull of its own gravity. It continues to shrink, becoming ever smaller and denser until its gravity becomes so great that nothing can escape form it, not even its own light. It has dug its own grave- a back hole,
Since a black hole is by definition invisible, it is of only academic interest to amateur observers. However, professional astronomers have detected X-ray emissions from various source which they believe are caused by hot gas plunging into the bottomless pits of black holes. The best-know candidate for a black hole, Cygnus X-1, orbits a 9th-magnitude star in the neck of Cygnus, the Swan.

The Expending Universe

In 1929 the American astronomer Edwin Hubble made the most significant of all discoveries in cosmology: the galaxies are moving apart from one another, as thought the Universe is expanding like a balloon being inflated. However, clusters of galaxies such as the Local Group do not expand they are held together by their natural gravitational attraction.
Hubble's discovery that the Universe is expanding came from a study of the spectrum of each galaxy's light. This revealed that the light form the galaxies was being lengthened in wavelength as result of high-speed recession (this called the Dopper effect). Such a lengthening of wavelength is called a redshift, because the light from the galaxy is moved towards the red (longer wavelength) end of the spectrum.