# How would one know whether a star would be observable?

• I
• Barely_Conscious
In summary, the question is whether a star with coordinates (R.A. 02:00:00, Dec. 05:00:00) will be above the local horizon between the sidereal times of 11:00:00 and 16:00:00 at a latitude of 50 degrees. It is assumed that if the R.A. is within 6 hours of either time, the star will be visible at some point, meaning it would be visible between 05:00:00 and 22:00:00 in this case. The Dec. needs to be between 90 and -40 degrees to be visible, and it is possible to use planetarium software to determine visibility for at least one hour under

#### Barely_Conscious

I'd like to answer this yes or no question for a number of objects: "Is this star, at any point between these two times, going to be above the local horizon?".

Say, I'm at the prime meridian at a latitude of 50 degrees, and I want to know whether, between the sidereal times of 11:00:00 and 16:00:00 (already calculated from GMT with appropriate corrections), a star of co-ordinates (R.A. 02:00:00, Dec. 05:00:00) would ever be above the horizon, how would I go about working it out?

I'm assuming (correct me if I'm wrong) that if the R.A is within 6 hours of either time, it will be above the horizon at some point, so in this case it'd be between 05:00:00 and 22:00:00 (this star wouldn't be above the horizon). As for the Dec., does it just have to be between 90 and -40 degrees to be visible? Or is there some other criterion I'm missing?

In addition to this, is it possible to determine, for each visible object, whether they'd be visible for at least one hour under 1.5 airmasses?

The simplest way would be to use a planetarium program set to your co-ordinates. Several are available free via the internet or a nominal cost. Search "free planetarium software".

Regards Andrew

## 1. How do scientists determine the visibility of a star?

Scientists use various tools and techniques to determine the visibility of a star. One method is to measure the star's apparent magnitude, which is its brightness as seen from Earth. Stars with a lower apparent magnitude are brighter and more easily observable. Another way is to use a telescope to observe the star's location and determine its position relative to other stars in the night sky. This can help determine if the star is visible from a particular location on Earth.

## 2. What factors affect the observability of a star?

Several factors can affect the observability of a star, including its distance from Earth, its size and brightness, and any obstructions in the night sky, such as clouds or light pollution. The time of year and location on Earth can also impact the visibility of a star, as some stars are only visible in certain seasons or from specific latitudes.

## 3. Can all stars be observed from Earth?

No, not all stars can be observed from Earth. Some stars are too far away to be seen with the naked eye or even with telescopes. Other stars may be too faint or small to be seen from Earth. Additionally, the Earth's atmosphere can distort or block the light from certain stars, making them difficult or impossible to observe.

## 4. How does a star's lifecycle affect its observability?

A star's lifecycle can have a significant impact on its observability. Younger stars, such as protostars, are often shrouded in gas and dust, making them difficult to observe. As a star ages and evolves, it may become brighter and more visible from Earth. However, as a star reaches the end of its life and becomes a white dwarf, it may become too faint to be observed.

## 5. Can the observability of a star change over time?

Yes, the observability of a star can change over time. As stars evolve, their brightness, size, and position in the night sky can all change, affecting their observability. Additionally, changes in Earth's atmosphere, such as weather patterns or changes in air pollution, can also impact the visibility of stars. Scientists continue to monitor and study stars to better understand their behavior and changes in observability over time.