A Federal Employment is a contract between two parties, one being the employer and the other being the Government as employee. In a commercial setting, the employer conceives of a productive activity, generally with the intention of creating profits, and the employee contributes labour to the enterprise, usually in return for payment of wages. Employment also exists in the public, nonprofit and household sectors.
In the United States, the "standard" employment contract is considered to be at-will meaning that the employer and employee are both free to terminate the employment at any time and for any cause, or for no cause at all.
To apply for one of those government jobs it is necessary to contact directly the U.S. Office of Personnel Management's USAJobs service, and many can be applied for online through their website.
Traditionally, these are one of the common employers of astrophysicists. The most frequent employers have been NASA and the Department of Energy (DoE). Other departments that might be possible employers are those run by the armed forces (Dept. of Defense Labs), the Department of Commerce (e.g., GFDL, NIST), and even the National Institute of Health (NIH). Over the past few decades, NASA's budget has been relatively flat. This is important to know because new funding means new jobs which are the most common filled by new Ph.D.'s.
The DoE has also not grown very much in total size but has undergone some significant shifts in funding which have resulted in many new jobs, particularly in computational science. Traditionally, the DoE has been in charge of the manufacture and development of the US nuclear weapons arsenal. It also has a large basic research component and has expanded its mission to include anything that falls under the umbrella of "national security", which may mean military, economic, domestic, etc. Many of the DoE labs require a security clearance, some don't.
The Office of Personnel Management or OPM is the United States government agency which serves to manage the civil service of the United States by the recruitment of qualified personnel into and the administration of their careers as part of the United States Civil Service.
The OPM is the federal government's human resources agency. It ensures that the nation's civil service remains free of political influence and that federal employees are selected and treated fairly and on the basis of merit. OPM supports agencies with personnel services and policy leadership, and it manages the federal retirement system and health insurance program.
The OPM was originally founded as the United States Civil Service Commission by the Civil Service Act of 1883. It became the OPM by the Civil Service Reform Act of 1978.