ماده ما بين النجوم
أخواني هذا الموضوع ليس منقول وهو يتكلم عن الماده الموجوده ما بين النجوم
THE INTERSTEALER MEDIUM
is the material which fills the space between the stars
these regions have very low densities and consist mainly of gas (99%)
and dust (1%)
Approximately 15% of the visible matter in the Milky Way is composed of
interstellar gas and dust.
approximately 99% of the interstellar medium is composed of interstellar
gas, and of its mass, about 75% is in the form of hydrogen (either
molecular or atomic), with the remaining 25% as helium.
The interstellar gas consists partly of neutral atoms and molecules, as
well as charged particles, such as ions and electrons. This gas is
extremely dilute, with an average density of about 1 atom per cubic
centimeter. (For comparison, the air we breathe has a density of
approximately 30,000,000,000,000,000,000 molecules per cubic
centimeter.) Even though the interstellar gas is very dilute, the amount
of matter adds up over the vast distances between the stars. The
interstellar gas is typically found in two forms:
1. Cold clouds of neutral atomic or molecular hydrogen
2. Hot ionized hydrogen near hot young stars.
The cold clouds of neutral or molecular hydrogen are the birthplace of
new stars if they become gravitationally unstable and collapse. The
neutral and molecular forms emit radiation in the radio band.
The ionized hydrogen is produced when large amounts of ultraviolet
radiation are released by hot newly-formed stars. This radiation ionizes
the surrounding clouds of gas. Visible light is emitted when electrons
recombine with the ionized hydrogen
Interstellar dust is not like the dust that you might find under your
bed; it is made of very different substances. These dust particles are
extremely small, just a fraction of a micron across, which happens to be
approximately the wavelength of blue light waves. The particles are
irregularly shaped, and are composed of silicates, carbon, and ice.
When light from other stars passes through the dust, a few things can
if the dust is thick enough, the light will be completely blocked,
leading to dark areas. These dark clouds are known as dark nebulae. The
Horse head Nebula, seen to the left, is an example of this.
Light passing through a dust cloud may not be completely blocked,
although all wavelengths of light passing through will be dimmed
The extinction is caused by the light being scattered off of the dust
particles out of our line of sight, preventing the light from reaching
us. The amount that the light is dimmed depends upon a few factors,
including the thickness and density of the dust cloud, as well as the
wavelength (color) of the light.
Because of the size of the dust particles, scattering of blue light is
favored. Therefore, less of the blue light reaches us, which means that
the light that reaches us is redder than it would have been without the
This is due to the effects of relative movement between a light source
and its receiver.
This process is similar to those that make the sun red at sunset.
A dust cloud that is illuminated by star light, This is similar to the
blue sky we see, which is produced by sunlight scattered by the Earth's
is a region of dusty gas surrounding a star where the dust reflects the
starlight, making it visible to us