Most stars are born in a huge cloud of gas and dust, called a nebula. The story starts when the nebula begins to shrink, then divides into smaller, swirling clumps. As each clump continues to collapse, the material in it becomes hotter and hotter. When it reaches about 18 million F (10 million C), nuclear reactions start and a new star is made.

Nebulas can be different colors. The color comes from the dust in the nebula, which can either absorb or reflect the radiation from newborn stars. In a blue nebula, light is reflected by small dust particles. A red nebula is caused by stars heating the dust and gas.

This is one of three huge fingers of cool hydrogen gas and dust. At the top of this finger, hot young stars shine brightly among the dark dust. Eventually these stars will blow the dust away and become clearly visible as a new star cluster.

Not all nebulas are colorful. The black Horsehead Nebula is a cloud of cold dust and gas that forms part of the Orion Nebula. The horse’s head shows up against the red nebula behind it, which is heated by stars. Many stars have formed in the Orion Nebula within the last million years.

The Pleiades cluster lies in the constellation of Taurus. It is also known as the Seven Sisters, because up to seven of its massive, white-hot stars can be seen with the naked eye. There are more than 300 young stars in the cluster, surrounded by a thin dust cloud that shows as a pale blue haze.

V838 Monocerotis is a red supergiant star, located about 20,000 light-years away from Earth. In March 2002, this star suddenly flared to 10,000 times its normal brightness. The series of images below shows how a burst of light from the star spread out into space, reflecting off the layers of dust that surround the star. This effect is called a light echo. The images make it look as if the nebula itself is growing, but it isn’t. The spectacular effect is caused by light from the stellar flash sweeping outward and lighting up more of the nebula.

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Sweat is produced by dedicated sweat glands, and is a mechanism used primarily by the body to reduce its internal temperature. There are two types of sweat gland in the human body, the eccrine gland and the apocrine gland. The former regulates body temperature, and is the primary source of excreted sweat, with the latter only secreting under emotional stresses, rather than those involved with body dehydration.

Eccrine sweat glands are controlled by the sympathetic nervous system and, when the internal temperature of the body rises, secrete a salty, water-based substance to the skin’s surface. This liquid then cools the skin and the body through evaporation, storing and then transferring excess heat into the atmosphere.

Both the eccrine and apocrine sweat glands only appear in mammals and, if active over the majority of the animal’s body, act as the primary thermoregulatory device. Certain mammals only have eccrine glands in specific areas – such as paws and lips – warranting the need to pant to control their temperature.

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