The interstellar medium

The space between stars in a galaxy is not empty. This is the interstellar medium. It is filled with a gas of very low density. If low in most of the space that this density is lower than that of the vacuum produced in laboratories on Earth. For comparison, the air around us has a density of 1 g per liter (a liter of air weighs 1 gram) or 10 22 molecules per liter. The vacuum in the laboratory corresponds to 10 -13 g per liter, or 1 million molecules per liter. In the interstellar medium density drops to 10 -21 g per liter, or 100 to 1000 atoms per liter. Although the number of particles per volume is low, the weight of the material between the stars is very important, because the distances, and therefore the volume between the stars are enormous. Accordingly, the interstellar medium represents a fraction of the mass of a galaxy which is often higher than with the sum of the masses of all stars.

See below, see this long ago: the light does not move instantaneously in space. It spreads to 300 000 km per second. Thus a particle of light (a photon) radiation at the surface of the sun at 150 million km dishes for 500 seconds to reach us, or 8 minutes and 20 seconds. Thus the sun as there was 8 minutes and 20 seconds. And they say the sun is 8 minutes and 20 seconds the lights of earth. The more we look away and we see the stars as they were in the past. Thus, the star nearest to Earth is about forty thousand billion kilometers or 40 pétamètres. Its light takes 4.22 years to reach us. The Andromeda galaxy, one of our neighboring rocks, is about 24 000 000 000 000 000 000 km or 24 zettamètres. His light is 2.54 million years to reach us. This means that if we see a star born "now" in Andromeda, it is born there are 2.54 million years. If it is very massive, it is highly likely to be already dead in a supernova explosion as we are witnessing its birth. As the universe has a beginning, we can not observe events that took place before it. The age of the universe is estimated at 13.7 billion years, we can not see farther than 13.7 billion light years is our horizon. This does not mean that the universe is not bigger, it simply means that you can not see beyond. It's already a lot, since this corresponds to 130 000 000 000 000 000 000 000 km or 130 yottamètres.

The evolution of galaxies

Herschel will allow us to measure unambiguously the stars born in the universe at every moment of its history, there are 10 billion years to the present day. Herschel will study in detail the composition of the interstellar medium of galaxies, focusing especially on interstellar dust that hides the birth of stars.

The different types of galaxies 

A galaxy is a collection of stars, gas, dust and dark matter. Taken together, the dust and gas form what is called the interstellar medium. There is a wide range of sizes and shapes of galaxies: 

1. Elliptical galaxies, gas poor, who form a fairly regular ellipse. 
   
2. Spiral galaxies, which are made of a hard end filled with a mixture of gas, dust and stars, and a thicker bulb in the center mainly filled with stars. It is in the disk of spiral arms qu'apparaissent more or less numerous and more or less marked. The Milky Way belongs to this category. 
   
3. Irregular galaxies, often small and low-mass gas-rich and-poor stars. 

These categories include the dwarf galaxies. The properties of dwarf galaxies are important because astronomers suggest that the most massive galaxies like ours come from the merger of small galaxies.

The Herschel Space Telescope Opens a first look at the universe

On 14 June 2009, the Herschel space telescope, then located in more than one million kilometers from Earth, was his first look at a galaxy. This image of the universe in the infrared submillimetre with an unprecedented fine resolution, was captured by the camera of the instrument bolometers PACS, designed and conducted by the CEA.

Medical tourism: how to control it?

Seek treatment abroad and enjoy the benefits of another health system is a practice becoming more widespread. In Europe, a proposed directive to regulate this new type of tourism.

Anne Laude

Previously, travel formed the youth today, some give a second. Indeed, journeys are now organized so that travelers receive care abroad. This phenomenon of medical tourism exceeds the requirements of spa treatments, because these are "travel for health care" to avoid long waiting lists, to benefit care prohibited in its country of origin or cheaper . In all cases, patients seeking to take advantage of the existing system of care in another state. Recognizing the problem, the European Commission is trying to legislate. We will see what measures it recommends having outlined the contours of the phenomenon. 

A recent example has made the headlines. On the night of Saturday 6 to Sunday 7 September, a 59-year-old woman gave birth to triplets in a Paris hospital. The mother has received a donation of oocytes in Vietnam. Why go so far? Because in France, oocyte donation is framed by the bioethics laws that impose a voluntary donation, anonymous and free. The recipient must suffer from infertility or pathological risk of transmitting a serious disease to the child. In addition, the couple must be of childbearing age, ie that the mother must be under 40 years. In France, couples wait 6 to 60 months before benefiting from this donation. Thus, going to Vietnam, the mother, on the one hand, benefited from more flexible conditions surrounding the gift, and on the other hand, won a few months.

The concept of tourism, associated with pleasure, therefore significantly changed. Increasingly, tourism is very specific purposes, whether cultural, scientific and medical today. Is not there some irony to involve tourism to something a priori binding?

In any case, the phenomenon is not ...