USAAAO — USA Astronomy and Astrophysics Organization
The national selection program that trains and chooses the United States team for the IOAA. Hundreds of students take a free first-round exam each year, with top scorers advancing through the National Astronomy Competition and an online training camp.
- Founded
- 2015
- Host rotation
- National (United States), exams held online and at testing sites
- Level
- National (United States)
- Format
- Progressive selection exams followed by a training camp
- Rounds
- First Round, National Astronomy Competition (NAC), training program, final team selection exam
- Eligibility
- US citizens or permanent residents enrolled in US secondary schools, within age limits for the IOAA
- Website
- usaaao.org
Notable facts
Entirely volunteer-run by students and alumni of olympiad programs. Its teams have earned multiple gold medals at recent IOAA editions, including at IOAA 2024 in Brazil.
Recent editions
| Year | Host |
|---|---|
| 2025 | Online (national) |
| 2024 | Online (national) |
| 2023 | Online (national) |
Typical topics
Past problems
2025
Einstein radius of a microlens
A 0.4-solar-mass star at 4 kpc lenses a background source at 8 kpc. Compute the angular Einstein radius in milliarcseconds and the expected event timescale for a relative transverse velocity of 200 km/s.
Show solution
The physical Einstein radius is R_E = sqrt(4GM/c² · D_L(D_S−D_L)/D_S). With M = 0.4 M_sun, D_L = 4 kpc and D_S = 8 kpc the reduced distance is D_L(D_S−D_L)/D_S = 2 kpc, giving R_E ≈ 5.1 AU. The angular radius is θ_E = R_E/D_L ≈ 5.1 AU / 4 kpc ≈ 1.27 mas. The event timescale is t_E = R_E/v = 5.1 AU / (200 km/s) ≈ 44 days.
2024
Masses of a spectroscopic binary
A double-lined spectroscopic binary shows radial-velocity amplitudes of 120 and 180 km/s on a circular 9.5-day orbit seen edge-on. Apply Kepler's third law and the mass–velocity relation to find both stellar masses in solar units.
Show solution
Seen edge-on, sin i = 1, so K1 = 120 and K2 = 180 km/s are the true orbital speeds. The mass ratio follows directly: M1/M2 = K2/K1 = 1.5. Kepler's third law for the relative orbit gives M1+M2 = (K1+K2)³P/(2πG); with K1+K2 = 300 km/s and P = 9.5 d, M_total ≈ 26.6 M_sun. Splitting by the ratio: M1 ≈ (1.5/2.5)·26.6 ≈ 16.0 M_sun and M2 ≈ 10.6 M_sun.