What is a Virtual Observatory? – An Overview

Our Universe is an incredibly complex place: large distance, long time scales and lots and lots of different objects ranging from grains of dust to enormous galaxy clusters.

Astronomers use many highly sophisticated techniques to make observations of our Universe. Immense quantities of images and spectra are stored in digital form in archives across the world. However, the data are all different of nature - they come from different telescopes and wavelength regions, making it difficult to bring data from different instruments together. There is an urgent need for tools that will allow astronomers to make sense of the flood of data, to avoid duplication and to extract as complete a picture of the Universe as possible.

The Virtual Observatory can streamline the once cumbersome process of searching for and collating astronomical data and bring together information to create a whole greater than the sum of its parts. This will be made possible partly by a huge effort to standardise the data as well as the methods and tools used by astronomers, and partly by using GRID technology: to access the great computational power at remote data centres across the world directly from the astronomer's desk.

The Real World

The real physical Universe is as complex as we can possible imagine, and then some. Imagine that we could travel into a galaxy. We would see structure in ever finer detail as more and more objects appear. We could in principle continue this zoom through several more stages until we reach the smallest sub-atomic building blocks we know, many orders of magnitude smaller than our starting point of a single galaxy.

An illustration of the complexity of the real world. Travelling into a galaxy we encounter ever more detailed structures.

Different observations produce different representations of the reality
When we observe the Universe we see one particular view or representation of reality. Our depiction depends on the telescope we use and whether the detectors inside the instruments are sensitive to visual light, or for instance radio or X-rays.

An illustration showing how different telescopes image the galaxy in the illustration above. A telescope taking an image in visual light (left), in radio (middle) and in X-rays (right).

Getting the full picture

When a doctor examines a patient's lungs he uses different methods and tools: X-ray images, listening to the lungs etc. In the same way astronomers need observations made at different wavelengths with different telescopes to give a better picture of what really is going on inside the object.

Without a Virtual Observatory

To combine different observations astronomers normally have to match the different coordinate systems in the images and then make their sizes and scales match. This is a difficult and time-consuming job, and prone to error since the data are often so different in nature.

Two different observations of the galaxy Messier 99 (Digitized Sky Survey on the left, Hubble Space Telescope on the right). The images have different scales (or 'magnification') and their orientation do not match. Also the co-ordinate system used may be different. If astronomers want to combine different images, they have to manually align and adjust them.

Before the scientific information can be extracted the astronomer will clean, calibrate and analyse the observations. Each different telescope and instrument produce characteristic artefacts that have to be removed.

Analysing an image. The astronomers search for certain objects in an image.

Depending on the interest of the astronomer he will perhaps search for certain objects. Maybe he will make catalogues over the objects,or and often he will compare his findings with existing catalogues in journals and on the Internet.

Three random catalogues with information about the galaxy Messier 100. Different quantities in catalogues, for instance brightness of a galaxy, are often measured in different ways and are therefore difficult to compare when taken at face value.

This work is difficult and has to be done by hand by an expert. There are thousands of catalogues, each with thousands or millions of objects. An object may have many different names and as an example the exact co-ordinates of its position often differs from catalogue to catalogue.

The astronomer has to find the information he needs by hand in catalogues in journals or electronically.

With a Virtual Observatory

The Virtual Observatory (VO) tools and methods will, for instance, help to:

Some of the scientific results that should emerge from the Virtual Observatory:

A Practical example (source: The NVO Project)

Problem: What are the objects - and the physics - behind Gamma Ray Bursts?

  1. Observations
    A satellite observes a Gamma Ray Burst and transmits the approximate co-ordinates back to Earth.
  2. Catalogue search
    Within seconds the Virtual Observatory - working on a server at one of the centres - will search for and receive archive images and catalogue information from all known catalogues for that particular region of the sky stored at various locations all over the world.
  3. Relationship generation
    The Virtual Observatory cross-matches the sources in the different catalogues and identifies a complete list of all known objects in the field.
  4. Image processing and analysis
    By comparing images taken at different times, the Virtual Observatory can for instance identify normal - scientifically uninteresting - variable stars and the like.
  5. Visualisation
    The Virtual Observatory then prepares a finding chart with links to the available information about the objects and posts it all on a website. It notifies interested astronomers about this unique opportunity for making follow-up observations with other telescopes at other wavelengths.
  6. Learning
    Astronomers can update the information as new results come in.

Conclusion

The Virtual Observatory will be a specialised resource as useful to astronomers as the Internet is to people making generalised queries. It will bring an instant library of structured and standardised information to the desktop supported by access to huge computational power available remotely at data centres around the world, to enable new discoveries and perhaps even whole new types of discovery.