What do i see while looking an astronomical picture?

Lot of times people make this question while looking a picture. A legitimate curiosity; they usually want to know what they're looking at, the material composition of the object and its dimensions. Sometimes they are just puzzled from being able to see colorful objects while they can't see anything in the sky, even if using good binoculars. So, let's try to satisfy this curiosity.

Astronomical pictures in the end concerns two different kind of 'playfield', commonly called "planetary imaging" and "deep space imaging". The imaging techniques are different, even if there are generic common difficulties.

Planetary Imaging

Planetary imaging basically refers to the act of taking pictures at subjects present in our Solar system. Obviously this means the Sun and all the planets, even Pluto which, with a controversial decision in 2006, is no more considered a planet like it was before, but just a dwarf planet quite like Ceres, the other dwarf planet in the Solar System, orbiting between Mars and Jupiter. Anyway, planetar pictures are usually self explicative: one just have to recognize the planet and make personal considerations.

Someone at this point will have noticed that background stars never show up in planetary images. The fact is planets are much nearer to earth than the stars are, so, having the photographer to follow them while taking the image, a parallax movement is produced and the camera isn't able to get the stars to show up in the final imagine. In other words, planet movement is faster for our bare eyes compared to the one of the background stars, and even have a different path in the sky. The planetary imaging technique isn't well suited to track different objects with different speeds and paths.

Planets of the Solar System may seem a really distant reality, or an alien reality as well. But if we stop for a second and think at planets whit a different, more cosmic oriented point of view, we have to consider them as just out of the entrance door, way before getting in our courtyard!

That's why people are surprised when i say that 6 out of 8 solar system planets are clearly visible with bare eyes in the sky, not only at night, but sometimes even at sunset..

Planet proportions are respected. Distances aren't.

Deep Sky Imaging

Astronomical pictures representing deep space objects of any kind are certainly more esotic and particular. That's because -with little exceptions- this kind of pictures show objects which are invisible using good quality binoculars; others are barely visible using good telescopes and a lot more are totally invisible even using bigger telescopes .

The largest part of this objects CAN BE shoot at with a good reflex digital camera AND a telescope, because digital cameras have modern sensors, which are many times more sensible compared to our bare eyes; they also have the possibility to 'store' incoming light informations, remember and sum those information generating bright images, brighter than images our eyes can sand to our brain.

If during a clean night we look at the sky, we will see a certain number of stars. We can move to mountain, far from cities, and the number of stars we see will increase dramatically, even 100 times (!) Light pollution, that's why. Cars headlight, light poles, neon. Their diffused light raise the level of backgound luminosity, and faint star can't show up in the sky. If we go back to our mountain, we will be able to see them again. But even this is not enough to notice deep sky objects, with little exceptions.

As soon as we take a look 'inside' a telescope, we realize that our little faint stars are in effect quite bright. And also we can see a vast number of other stars, invisible to bare eyes. We will soon get lost. A wide filed picture of that part of the sky will surely show an impressive number of stars

Part of the Milkty Way. Shot taken with a common digital camera. - (c) Paolo Duzioni

Hidden in this sea of light we will find the most strange and exotic cosmic objects, of different colors and shapes: galaxies, nebulas, star clusters..

NEBULA

In astronomy, a nebula is a cloud of cosmic gas and dust in outer space, and is further divided in different categories, based on chemical composition and different kind of illumination.

OBSCURE NEBULA

Obscure nebula can be observed only if hiding another nebula or part of another nebula in the background, of if they block the background stars light. Obscure nebula aren't holes in the space as they seem, nor they are empty. On the contrary they are full of stars, but those are hidden from view and not noticeble.

Obscure nebula Barnard 68 - Credit FORS Team, ESO

EMISSION NEBULA

Emission nebula is a cloud of ionized gas, emitting ligth with different colors. The origin of ionizations are commonly high energy photons, emitted by a near hot star. Usually, a young star ionize part of the same nebula from which the star itself was born. Only big and hot stars can release a sufficient quantity of energy, needed to ionize a relevant part of the nebula. More often, an entire cluster of young stars does the job. The color of the nebula is directly depending by its chemical composition and the level of ionization.

Emission Nebula M42 Orion - (c) Paolo Duzioni

REFLECTION NEBULA

In astronomy, reflection nebula are clouds of dust reflecting the light of nearby stars, just like headlights of a passing car 'illuminate' the fog. Reflection nebula take form when near stars aren't hot enought to cause gas ionization like in emission nebula, but at the same time are enought bright to make the dust visible. Reflection nebula are usually a star formation zone, like the Pleiads cluster, young stars illuminating the surrounding dust, becoming a clear example of reflection nebula

Reflection nebula M45 Pleiadi - (c) Paolo Duzioni

PLANETARY NEBULA

Planetary nebula is an astronomic object usually appearing in a disk form, when observed at small/medium resolutions. Due to this appearance, similar to planets of first astronomical observations, "planetary" was the name given to this type of nebula. Reality is that those are the product of last steps in a star death. When a star is close to death, it ejects its outer layers in different episodes of nuclear fusion reactions. The star collapses in a white dwarf and the expelled gas forms a dust cloud around the star itself: this is the planetary nebula. The remainder of the star is responsible for the production of the energy that illuminates the nebula, the ultraviolet radiation. Depending of chemical composition and of radiation power, the nebula can assume red, green and even blue coloring.

Planetary Nebula NGC 6543 "Cat's Eye " - Credit NASA/ESA

GALAXIES

In astronomy with the tem 'galaxy' we are referring to a system gavitationally tied, made of stars, interstellar gases, dust and probably obsure matter, recently discovered -or, better said, teorized- as it's completely not detectable. Galaxies can assume irregular, ellittic or spiral forms. They usually tend to assemble in more complex structures like galaxies groups or the larger galaxies clusters.

Different kind of galaxies

An example of galaxy is our Milky way, where our solar system is collocated. Milky way is a barred spiral galaxy (SBC in the picture above) witha diameter of 100.000 light years, including about 400 billions of stars (or suns, if preferred) and with a total mass of one thousand billion times the mass of our sun.

Spiral galaxy M31 Andromeda - (c) Paolo Duzioni

Space between galaxies is void, except for interstellar gas clouds.

Few galaxies are isolated, most of them are gravitationally tied to lot of others. Structures containing up to 50 galaxies are named galaxy groups, while larger structures containing thousands of galaxies are called galaxy clusters. Galaxy superclusters are mega-structures containing dozens and dozens thousands of galaxies.

Our galaxy is part of the local group, and is one of the most massive, second just to the Andromeda Galaxy, more than 100.000 light years wide. The whole local group counts up to over 30 galaxies, with a total diameter of more than 10 megaparsec.

STAR CLUSTERS

GLOBULAR CLUSTERS

A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is derived from the Latin globulus—a small sphere. They are usually formed by hundreds of thousands stars.

Although it appears that globular clusters contain some of the first stars to be produced in the galaxy, their origins and their role in galactic evolution are still unclear. It does appear clear that globular clusters are significantly different from dwarf elliptical galaxies and were formed as part of the star formation of the parent galaxy rather than as a separate galaxy.

Globular cluster M80 - Credit STScI

OPEN CLUSTERS

An open cluster is a group of stars, all generated toghther from a giant molecular cloud, and still held togheter by a strong gravitational attraction. They are also named 'galactic clusters', because can be found only inside galaxies. Open clusters are young objects and therefore are formed by young and shiny stars. Open clusters are detectable from far distances. The 'mother' molecular cloud is sometime still associated with the cluster, and illuminated from it, generating a nebula, like the Pleiads open cluster.

Stars forming open clusters are initially close one each other, moving inside the galaxy at the same speed. After about half a billion years, open clusters are usually disturbed from external objects passing nearby; the stars start moving with sligthly differents speeds and over time the cluster became like a' stars flow'. After a billion years, it has completely vanished. Some stars can be found at opposite positions inside the galaxy. The more massive is the open cluster, more time will be required to disgregate.

Open cluster NGC265 in Magellan. Credit NASA/ESA.

In other words in astronomic images we can see different kind of light manifestations, originated by stars' chemical reactions, illuminating other space matter. We can see other cosmic matters, reacting each other, originating solid bodies, gas clouds; attracting other bodies, changing form and dimension in time. In perfect balance, or perpetual chaos, depending on our point of view .

Descibing dimensions and distances using numbers is quite useless, we will surely lose ourselves in a sea of numbers.

The following clip may help us to understand that in a cosmic way, our earth is not much bigger than a nut.

A small one.