# Different kinds of the same isotope

I apologize if this is an obvious question, but I can't find the answer anywhere. In this page: http://ie.lbl.gov/education/parent/U_iso.htm are listed the isotopes of Uranium. Some of them, for example U238 are listed many times, with "m1" or "m2" attached to the mass number. What does this mean?

• Quick Google search led me to wikipedia's article on metastable isomers. Notation is given there. – Kyle Kanos Jan 18 '14 at 17:00
• Thank you! I used google too and found nothing... weird. – carllacan Jan 18 '14 at 17:23

• I just can't resist adding the truly remarkable example of why not all metastable states decay quickly: Tantalum-180m. It's the only naturally-encountered excited nucleus present in macroscopic quantities on Earth, and it is stabler than its ground state by an incredible factor of at least $10^{18}$. – Nicolau Saker Neto Jan 18 '14 at 20:36